Frustrated with RX7 brakes!

Stop me if I've posted in the wrong location. I'm at a complete loss when it comes to the brakes on my RX7. I apologize for the novel. I like details when problem-solving so I try to lay everything out.

TL/DR:

I refreshed the entire brake system: new master cylinder and hoses, cleaned and painted booster and cleaned proportioning valve. Now I have no brakes despite being confident there's no air in the system. I'm stuck.

Long version:

This is somewhat embarassing because automotive braking systems and their maintenance is something I thought I understood fairly well. Over the Summer I've been refreshing my '79 RX7. It's over 42 years old and as far as I can tell most everything is original and that includes all the rubber bits. I'm getting around to correcting that starting with the most import things: coolant hoses, fuel lines, clutch and brake hydraulics, etc. New clutch master and slave cylinders and rubber line went in without a hitch. Last weekened I decided to tackle the brakes. Friday night I swapped the three rubber lines and bench bled the new master cylinder. All Centric aftermarket parts. I also adjusted the shoes in the rear so the drums were *just* able to move freely. Saturday I went about removing the old MC only to be greeted with a brake booster that looked like this:

To be clear, I found no evidence of the MC leaking but the service records from the previous owner indicate it was replaced back in '88 so perhaps this is leftover from that. Since I was in there I decided to freshen up the booster. A few hours with degreaser, sandpaper and some VHT epoxy paint and I have this:

Waited a day for the paint to cure then installed everything on Sunday. Looks clean(er):

I then initiated the bleeding process. I have a Motive pressure bleeder that has made work on my other cars ('86 RX7, '10 Jetta, etc.) almost comically easy. Hooked up everything, pumped the system to 15 psi and bled everything. The RX7 is a little weird in that there's only one rear bleeder in the rear on the RR wheel cylinder and there's also a bleeder on the MC itself. I bled RR, FL, FR and then finally at the MC again. Even after using the Motive tool I still had my oldest help me with the old-fashioned way: pump pedal, hold pedal, crack bleeder, wait a second or two, close bleeder before fluid stops then release pedal. Did this in the same order as before. I like to do this when I've replaced components (e.g. MC, calipers) because it generates significantly more pressure than the Motive tool.

Confident in a job well done I bolted the wheels back up, torqued them to spec, lowered the car and got ready for the post-job test drive. Brake pedal felt fine - as good as a 42 year-old car with drum brakes can feel. Here's where the problem starts. I fire up the engine and the pedal goes straight down. Pumping does nothing. To be clear, the pedal isn't going all the way to the floor. It eventually hits some limit. Be it the end of the cylinder of the MC or I finally get hydraulic pressure, I'm not sure. At this point the wheels are locked and cannot be turned by hand. However, I don't feel the car is safe to drive so back up in the air it goes. I spent the rest of Sunday afternoon looking for leaks, buldges, stuck pistons/shoes, you name it. I bled the brakes again yet I'm left with the same result. It's now getting late, I'm beat so I admit defeat and call it a day.

My first thought is maybe I got a bad "new" MC. I understand most of these are actually rebuilds and sometimes you get unlucky. I've had bad rebuilt calipers and MCs before. I decided to order another MC using the OEM PN from a reputable rotary parts dealer at a premium price point. I also ordered some new brake fittings so I can ensure I've bled the MC properly. I've got some cutup pieces of old brake hard line I use to attach the tubes for the bleeding process. The problem is this car's fittings are M10x1.25 whereas all the fittings I had were M10x1.0. I also ordered some M10x1.25 to -3AN adapters and some -3AN plugs so I could verify the MC was bled and confirm a hard pedal once installed on the car. Since I ordered another MC I decided to tear down the two I had and look for differences. At first glance the only noticeable difference was the resvoirs:

The new MC has the cap in the middle and the front and rear sections appear to be the same volume. The old one has the cap nearer the rear and there's more volume in the front section of the resevoir. Other than that, externally, they are the same. Number of ports, port locations and external dimensions are the same. Once I started the disassembly process I noticed a couple more differences:

The first is the primary piston retention method. The new MC uses a simple circlip whereas the old has a metal washer and a wierd triangular spring clip. As far as I can tell they both seat the same depth in the bore. The other obvious difference is the old MC pistons are clearly cast aluminum while the new one's are machined Al. Bore and piston diameter, piston length, spring free length, etc. are all within manufacturing tolerances of eachother.

Now I'm thinking the MC is not likely at fault but I'll still wait for the "OEM" one to come in. The other major component I touched was the booster. I know when these fail it usually results in a hard pedal, not soft. Regardless, I start looking for obvious faults. The booster holds 20 in. Hg vacuum just fine. The pedal definitely sinks when the engine is started. Pumping the pedal after the engine is shut off gradually results in a firmer pedal once the internal vacuum is released. I did find one thing. The OEM spec for booster pushrod-to-MC clearance is 0.1-0.5 mm. Mine was like 5 mm and confirmed with both MCs. That seems odd considering this would be a pretty big miss at assembly and no one should have touched this since. This could certainly account for some of my excess pedal travel. I get the pushrod clearance set as well as check and set the pedal height and freeplay to OEM specs. Over the week parts start to trickle in.

By early afternoon Saturday everything has arrived. The new, new MC looks identical to the old, new MC except it came with another fluid level sensor. I bench bleed the MC. First without the proportioning valve then again with it installed. With all the ports plugged I get less than 1/8" piston movement on the bench. I installed the MC on the car and with the ports still plugged I get maybe 1/2" pedal movement after the pushrod engages the MC. I attribute this to mechanical advantage of the pedal. Next, I start the bleeding process. I use my Motive pressure bleeder again but this time I bleed connect and bleed each circuit one-by-one. First I connect the rear circuit, bleed it, then I connect the FR circuit, bleed it and so on. Once I'm done with the Motive tool I do it the old-fashioned way again, but with a twist. The Factory service manual gives a slight variation: open bleeder, depress pedal all the way, allow pedal to return to full height, then close the bleeder. Repeat until bubbles are gone. I do this RR, FR, FL and MC. Hop in the car and the pedal feels better, but that might just be due to the reduction in pushrod clearance. Put the wheels back on, lower the car, fire it up AND THE PEDAL GOES TO THE FLOOR AGAIN! Against better judgement I take the car for a quick spin and it's as bad as you would expect. Essentially no brakes. However, if I stomp on the pedal hard/fast enough something does lock up. Felt like the rear. Back at home I plugged the ports on the MC again and confirmed I still had ~1/2" of pedal travel. However, when the engine running that 1/2" has no resistance.

So this is where I am. I've used two different "new" MCs. I've bled everything more times than I can count with three different methods. There are no leaks. There are no bulging brake lines. Rear shoes are adjusted probably tighter than they should be. I even replaced the RL wheel cylinder because the shoes were moving unevenly and I had a spare on hand. As far as I can tell everything is in spec. Here's a list of everything I've touched/adjusted:

1. Replaced rubber brake hoses at rear and FL and FR connections with aftermarket Centric parts. No leaks from connections and no bulging with the pedal depressed.

2. Replaced MC twice, once with Centric part and again with "OEM" part. Bench bled both times. Piston moves ~1/8" with ports blocked on bench. No leaks at connections.

3. Removed, cleaned and painted the brake booster. Holds up to 20 in. Hg vacuum. Adjusted pushrod-to-MC clearnace to ~0.5 mm. Pedal goes soft when car is started goes hard again after a few pumps with engine off.

4. Removed and cleaned the brake proportioning valve. No leaks at connections.

5. Adjusted rear brake shoes to *just* drag on the drums.

6. Replaced RL wheel cylinder. No leaks at connections.

7. Adjusted brake pedal height and free play to OEM specs.

I'm at the point now where I'm going to start going back and putting the old parts back on. I've already disassembled, cleaned and reassembled the old MC. The next step is to bench bleed and install that and try again. If that doesn't work that pretty much leaves the brake booster or proportioning valve. Maybe I damaged something during the cleaning process. I'm trying to track down some used parts to test. Lines are lines to me so unless there's visible leaking or buldging I assume their doing their job.

Sorry for the novel. So what should have been a simple, straightforward job on an old car has turned complicated. I'm completely stumped.

I have had the masters on these cars leek internally causing the fluid the passing the piston when under pressure resulting in no external leak but also no pressure in the system and the peddle goes to the floor.  It works enough to bench bleed the master but leaks by when you put "real" pressure on it. 

The other thing i have found is I think there is a difference in the masters between either early and late FB cars and or standard FB cars and the GSLSE.  I had to trim down the push rod on one car in order for it wo work with the new master I was putting in it.  I think the difference also applies between the FC NA cars and the FC turbo cars.  I ran in to the same problem swapping over a t2 drivetrain in to an 86 sport.  I never figuared if it was a difference in the masters or with the booster.

have you been able to bench bleed the MC? 

" The RX7 is a little weird in that there's only one rear bleeder in the rear on the RR wheel cylinder and there's also a bleeder on the MC itself. "

Um, what? How the hell do you bleed the other side of the axle? Sounds like a massive air bubble at that wheel end.

Looking on Rock Auto, both rear wheel cylinders are the same?

Also, i don't know if it will matter, bleed right rear, left rear, right front and then left front. The shortest run should be last. It looks like you went right rear, left front and then right front which could have something to do with it depending on where it goes after the master cylinder.

In reply to fidelity101 (Forum Supporter) :

The OP did mention bench bleeding both m/cs.

kevinatfms said:

" The RX7 is a little weird in that there's only one rear bleeder in the rear on the RR wheel cylinder and there's also a bleeder on the MC itself. "

Um, what? How the hell do you bleed the other side of the axle? Sounds like a massive air bubble at that wheel end.

I scratched my head at that arrangement when I was looking at my Maxton that uses an FB rear axle. Basically the LR wheel cylinder has two hard lines, one connected to the m/c side of things and the other running across the axle to the RR wheel cylinder, which is the one that has the bleed screw.

In reply to dean1484 :

That's what I thought might have been the case with the first new MC since I didn't have the tools to test it. I ultimately pulled that one apart but nothing looked out of the ordinary. For the second new MC I bought M10x1.25 inverted flare to -3AN male adapters and some -3AN female caps so I could block all the ports after bench bleeding. After bleeding, on the bench I could only move the piston ~1/8" pushing with all my might. Installed on the car, but still plugged, the brake pedal would move maybe 1/2" and I figure this is due to the small pushrod-to-MC clearance (~0.5 mm) and mechanical advantage of the pedal. If the MC was bypassing internally, wouldn't I be able to feel that in either situation?

I know the brake systems on these cars changed quite a bit. The MCs are unique for 79, 80, 81-83 with rear drums, 81-83 with rear discs, and 84-85 with rear discs. The first new MC I bought is supposedly the correct Centric replacement for a 79. The second new MC was advertised as being for 79-80 but Mazda and some aftermarket companies specify different PNs. I'll admit I have no idea what's different between 79 and 80 other than PN.

I'm well aware of the FC N/A vs TII brake differences. My 86 base model has the upgraded brakes and a 929 MC/booster combination (same style as TII and FD).

In reply to fidelity101 (Forum Supporter) :

Yep, each time. The first time wasn't ideal since I didn't have the correct fittings. I ended up having to use the two small hard lines to the prop valve so I could only bleed two ports at a time. Even then I wasn't able to verify it was bled. The second time I got some M10x1.25 inverted flare fittings and connected clear vinyl tube to some cut pieces of old hard line. The tube was looped back into the reservoir, front ports to front and rear port to rear, and the pistons were cycled until no more bubbles came out. Interestingly enough, I followed this up by bleeding the MC using the valve on its side and got a few more bubbles out. After that I plugged all the port and attempting to move the pistons. Trying as hard as I could, I could only move it ~1/8" on the bench. Installed on the car with the ports still plugged I get about 1/2" of brake travel after the booster pushrod engages the MC. This is the same process I've used for years on my FC without issue, which is why I'm so stumped. It's doesn't appear to be a MC issue but that's the only major component I changed.

In reply to kevinatfms :

There is most likely only one brake line to the rear. I've seen where the brake line goes to one wheel cylinder and then instead of a bleed screw another brake line goes from that wheel cylinder across to the other side which does have a bleed screw.

The Ford/Mercury Capri of the 70's also used this set up.

without being able to troubleshoot it in person, my best guess is that the piston seals in the new masters are not sealing under the much higher force they see when the booster is boosting.  this happens a lot more than it should.  my lifetime "first part" success rate on new aftermarket masters is about 40%, meaning 6 out of 10 had to be returned *at least once*. by far the worst brand in my experience is A1 Cardone, absolute dog E36 M3, returned 3 same PN before getting one that held pressure in both chambers.

In reply to kevinatfms :

Yeah, like Tim said the wheel cylinders are unique. The RL has two regular inverted flare ports: one goes to the proportioning valve and the other goes to the RR wheel cylinder, which has one inverted flare connection and one bleeder port. It doesn't seem ideal to me either, since I'm used to dealing with rear disc brakes that have a tee, but it shouldn't really matter. Any air upstream of the RL wheel cylinder should get pushed through it over to the RR with fluid.

I've bled/flushed the fluid in the car many times before without issues. The fact that my brake fluid always turned brown after a couple of weeks is why I ultimately decided to replace the MC. I did confirm after I replaced the RL wheel cylinder that both shoes were moving evenly and at the same time. Before that one shoe would move a bit and then the other would. I don't know if that indicates an issue or not. The RR side does this as well but not as pronounced.

In reply to AngryCorvair (Forum Supporter) :

I don't know how much the booster amplifies the force but that's a good theory. It's supported by my observations that the pedal seems okay with the engine off but immediately sinks when the it's started. Looking at my picture above there does appear to be a slight difference in the rear-most seal on the primary piston. On the new part the aft end of the seal appears to be smaller than the piston whereas on the old part it's a similar diameter:

Whether this is a design flaw or the old part has swelled due to years of exposure to brake fluid is unknown. I didn't disassemble the second new MC but I assume it's similar to the first. The first is from Centric, which I consider to be one of the better brands. Behind OEM and Beck Arnley in my hierarchy. I agree with your sentiments on A1 Cardone. I'm not sure who made the second new one. I ordered it from a well-known rotary parts dealer using the OEM PN. I'm kind of annoyed because I paid a 3X premium for what looks like the same part. Even my old MC isn't original. My records show it being replaced in '88 but at least it has the same construction as shown in the FSM. I had some free time this morning and got it bench bled and so far it feels the same. If using it ends up fixing the issue I'll be glad but at the same time concerned. These things don't last forever, this one is already 32 years old with issues (discolored brake fluid) and apparently it means "new" ones are really unreliable.

infernosg said:

In reply to kevinatfms :

Yeah, like Tim said the wheel cylinders are unique. The RL has two regular inverted flare ports: one goes to the proportioning valve and the other goes to the RR wheel cylinder, which has one inverted flare connection and one bleeder port. It doesn't seem ideal to me either, since I'm used to dealing with rear disc brakes that have a tee, but it shouldn't really matter. Any air upstream of the RL wheel cylinder should get pushed through it over to the RR with fluid.

I've bled/flushed the fluid in the car many times before without issues. The fact that my brake fluid always turned brown after a couple of weeks is why I ultimately decided to replace the MC. I did confirm after I replaced the RL wheel cylinder that both shoes were moving evenly and at the same time. Before that one shoe would move a bit and then the other would. I don't know if that indicates an issue or not. The RR side does this as well but not as pronounced.

Thats what i assumed but it sounded so odd. Ive worked on a few oddly design brake systems but that just sounds so counterintuitive to conventional thinking.

Gotta love some manufacturer's who try to keep it simple it but end up making it more complicated than it needs to be.

The calipers were not replaced correct? Just checking to make sure they aren't on the wrong side with the bleeders facing down.

I would adjust the rear shoes til the drums won't turn and bleed the brakes again. Then back off the shoes til the drum turns but with some drag. Drive it and see.

Bad new parts happens. When it does you start replacing everything else, knowing that the first one you changed is new so it has to be good. But when its not, makes for a real nightmare.

In reply to sergio :

Correct. The only items replaced were the rubber brake lines, the MC and RR wheel cylinder. The front calipers are on my list. They're the old, weird sliding wedge design. Going to have to rebuild those myself but it doesn't look bad. Good idea on the shoes. I'll try that after I put the old MC back in.

One other thought - take the drums off and see if there are traces of fluid under the outer rubber seals of the wheel cylinders. I've had that before where restoring the brake system to its supposed operating pressure caused the wheel cylinders to leak, but not enough to be immediately visible.

OTOH given the symptoms that you describe I would expect a brake fluid fountain if that was the case.

Get some brake line clamps and put them on the soft lines.  See if the pedal feels good with the clamps on there.

Remove one clamp at a time to determine which circuit it is.

I had a very similar issue with master cylinders on my 87 rx7.  Went through about 4 master cylinders until I put an old crusty one on that I got out of a junkyard and cleaned it up.  That was the only one that held pressure. 

In reply to wvumtnbkr :

I've done something similar but left the port plugged for each circuit at the MC instead of clamping the line. I think it's the rear circuit that causes the most movement but moreso because drums always seem to have a lot of travel no matter how tightly they are set.

I've got the old MC cleaned up, put back together and bench bled. Just need to find some time to put it back on the car. I've been monitoring it for a day now and it may have a very small leak at the rear. Hard to tell if it's actually leaking or if it's just a small residual amount of fluid from when I put everything back together. I've got my hands on another used OEM assembly (booster, MC, prop valve) to try in case the issue doesn't go away.

I still can't help to think this might be a prop valve issue. I didn't take it apart but gave it a good soaking in brake parts cleaner. On the short test drive I took I was able to briefly lock up the brakes at one point with a quick stab of the brake pedal. Not entirely sure which brakes locked up but it felt like the rear. My understanding is the prop valve is supposed to prevent this. Guess I won't know until I start testing.

I think you've got a leaky booster and it's pulling fluid from the master somehow.  See how the brakes work with the vacuum line to the booster disconnected and plug on the engine side.

In reply to APEowner :

Can the booster leak "internally" that wouldn't show up with a vacuum test? With a hand pump it's held 20 in. Hg with no issues. It does the normal thing where the pedal drops when the engine is started and pumping the brakes with the engine off again results in a progressively harder pedal. I think the only way the booster could be the culprit is if it's somehow "over-boosting" the system giving more assist than it should be. I'm not sure how that would be possible though.

infernosg said:

In reply to APEowner :

Can the booster leak "internally" that wouldn't show up with a vacuum test? With a hand pump it's held 20 in. Hg with no issues. It does the normal thing where the pedal drops when the engine is started and pumping the brakes with the engine off again results in a progressively harder pedal. I think the only way the booster could be the culprit is if it's somehow "over-boosting" the system giving more assist than it should be. I'm not sure how that would be possible though.

Hmm.  I missed the part about it holding vacuum with a hand pump.  I'm less confident in the booster being the problem than I was.  I guess it could leak at 30 in hg and not 20 but that would be weird.  On the other hand weird is what you've got. 

I think I'd still try driving it without the booster.  If I'm following your tale of woe correctly the system seems to work OK until the engine is running and I think it would be helpful to know if that's really the case.

wvumtnbkr said:

Get some brake line clamps and put them on the soft lines.  See if the pedal feels good with the clamps on there.

 

Remove one clamp at a time to determine which circuit it is.

 

I had a very similar issue with master cylinders on my 87 rx7.  Went through about 4 master cylinders until I put an old crusty one on that I got out of a junkyard and cleaned it up.  That was the only one that held pressure. 

You need to isolate where the problem is. You can use vise grips to lightly pinch the rubber flexible hoses if you have no brake line clamps. I like how you plugged the master cylinder hard line connection which is also a good way to isolate the problem.

Other than that I'm highly suspicious of the brake booster because the pedal drops to the floor with the engine running. I'd disconnect and plug the booster vacuum line and go for a careful drive to determine if the brakes function without the booster.Push hard on the pedal!

Good luck and nice car

Well, the symptoms persist. I disassembled, cleaned, reassembled, bled and reinstalled the original MC today. Brakes were then bled with the rear drums locked. Same problem. At this point here are the possibilities:

1. 3 bad MCs. "New" ones were bad out of the box and I damaged the original one.

2. Bad brake booster.

3. Bad brake proportioning valve.

4. Still air in the system.

Some more thoughts/observations:

-When I first install the MC on the car with all the ports blocked I get very little pedal movement, <1/2". I assume this is the <1/8" piston movement I get on the bench multiplied by the lever arm of the brake pedal. As I install each circuit I get a little more pedal movement each time. Up to around 1" total I'd guess. I assume this is due to moving the caliper/wheel cylinder pistons but it doesn't appear that any one circuit it worse than another. Any pedal movement I have, whether it's 1/2" or 1", is made effortless when the engine is started.

-With the engine running and the booster connected I cracked the bleeder on the MC. The pedal easily moves that first 1/2" or 1" then hits resistance and starts pushing fluid out of the MC until the pedal hits the floor. This feels like fluid is either bypassing one of the pistons completely, there's lots of air in one or more circuits or it's taking a lot of pedal travel to push enough fluid to move a caliper/wheel cylinder piston before the pad/shoe contacts the rotor/drum. However, it seems unlikely that (1) all the MCs are bad including the original, (2) there's THAT MUCH air in the system considering how much bleeding I've done and (3) there's a decent amount of brake drag at each wheel. This also tells me it's unlikely an issue of the booster "over boosting" since it does eventually hit resistance.

-I'm now focusing on the proportioning valve. My understanding is this thing makes the front and rear brakes see the correct amount of pressure because disc brakes require more force than drums to operate. If this thing is somehow bad and is directing an equal amount of pressure to all four corners that would explain poor braking performance and lockup of the rear but not necessarily a soft pedal. I was able to locate another used assembly. Who knows when that will all come in though.

Here's a thought: prop valves have a shuttle mechanism inside them that is displaced to one side or the other when there's a leak somewhere in the system. If you shuttled it during the bleed, it could be stuck in that position, and may not be allowing both circuits to flow properly. Do you notice a big difference in the amount of fluid coming out the bleeders at one end of the car vs the other?

In reply to AngryCorvair (Forum Supporter) :

I was trying to think of that myself. I didn't really pay attention to the flow rate, unfortunately. I did try rapping on the proportioning valve lightly with a small hammer in attempt to dislodge anything that might be stuck. I don't know if it's worth mentioning but I've been bench bleeding the MC with the proportioning valve connected. My process has gone like this: (1) bench bleed the MC by itself from the three ports, (2) plug the ports and bleed again using only the bleeder valve, (3) attach the proportioning valve and bleed it and the MC using the two open ports (forward-most port on the MC goes straight to the FL caliper so it remains plugged during this step), finally (4) bleed the whole thing again using the valve on the MC one last time. I don't think it'd make a difference but perhaps I shouldn't be bleeding the proportioning valve this way?

I've got everything off the car again. No signs of leaks from the back of the old MC fortunately. I've gone ahead and disassembled the proportioning valve as well. As with everything else I don't see anything obviously wrong. Sure there's some gunk and a few bits of rust but nothing that looks like it'd cause a problem. Neat piece of engineering. Going to clean it up and put it back together.

I too have a spark yellow SA22C. I've owned it for 15 years and it is basically all stock and original. I've found that the rear shoe adjusters play a huge role in how the brakes feel. Ii didn't read that book of a post. I'd make sure that the rears are adjusted properly. Basically I adjust mine until the rear drums don't move by hand (with wheels off) and go from there.