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Login to post Forums » Grassroots Motorsports » Brake line failure - Help me understand

May 2, 2011 1:44 p.m. Jamesc2123 Reader

So, the night before the first autocross of the season, my CRX managed to blow out one of the brake lines under the hood (!!!!!). Its one of two hard lines that go from the master cylinder to the proportioning valve. Frustrating, but I guess I can be thankful that it happened the night before and not going into the finish gate or something on Sunday...

That's all whatever, but help me understand this, because it goes against how I thought brakes worked: If the one line blew out, that should only take out just two of the brakes, right? The driver's left, and passenger rear, or vice versa. But, when that line went, I had NO brakes. Couldn't pump them, couldn't stop, NOTHING. I know that the two brakes on the other diagonal were functioning before because I had just bled them a day or two ago, and they were fine.

So why did I have no brakes? What am I not understanding?
May 2, 2011 2:05 p.m. failboat Reader

the whole system is pressurized, you open it up in one spot and you lose pressure for everything.

think about when you bleed brakes and you open one of the bleeder screws, once you get that pedal to the floor there is hardly any pressure in the system until you close the bleeder screw and pump the system back up so to speak. If theres an opening the fluid just going to travel the path of least resistance.....right out the leak in your system and not applying pressure to your brakes.
May 2, 2011 2:14 p.m. Jamesc2123 Reader

Yeah that makes sense. But still, I thought the master cylinder split brake pressure into two closed systems so that one could work if the other didn't..
May 2, 2011 2:21 p.m. tpwalsh New Reader

I'm not familiar with the crx braking system, but by law, all cars built after the late 60's are required to have dual circuit brakes. BUT a CRX has pretty crappy rear brakes. If it's split F/R vs. LF,RR/RF,LR and you lose the front brakes you might as well have no brakes.
May 2, 2011 2:30 p.m. failboat Reader

tpwalsh wrote:

....but by law, all cars built after the late 60's are required to have dual circuit brakes.

I didnt know that. I need to pay attention to this thread because I will learn something.

All I do know is you open a bleeder valve and brake system pressure drops to next to nothing, I imagine a leak in a brake line behaves teh same way. All the cars I have done brakes on are waaaayy newer than the 60's.
May 2, 2011 2:40 p.m. tuna55 SuperDork

failboat wrote:

tpwalsh wrote:

....but by law, all cars built after the late 60's are required to have dual circuit brakes.

I didnt know that. I need to pay attention to this thread because I will learn something.

All I do know is you open a bleeder valve and brake system pressure drops to next to nothing, I imagine a leak in a brake line behaves teh same way. All the cars I have done brakes on are waaaayy newer than the 60's.

I had a master fail on a dual circuit system and it had no brakes until the 15th pump or so scared pump. I presume a failed line would do the same.
May 2, 2011 3:19 p.m. ransom Reader

I had the swaged end come off of a braided stainless front brake line on my 2002 at an autocross, and had the same pedal-to-the-floor result... I was incredibly glad it was at an autocross, and not somewhere on the street resulting in injury/death.

And I still don't have a satisfactory answer about why I didn't retain at least RF/LR brakes (EDIT: I guess that'd be RF/RR brakes, since both rears are fed by a single line on this car). Haven't worked on it in a while, but the 2002 has two lines to each front caliper, IIRC one each from the front and rear pistons in the master, with the rear piston also feeding a single line which runs to the rear and is then T'ed to the drums.

Actually, if that's the case, and with single-piston calipers... When my line blew, it vented not only the pressure from that line's master piston, but would also have provided an outlet for the pressure from the other piston by way of the caliper body. I guess ruptured line wasn't a failure mode that was meant to catch...

What's the CRX's line routing?
May 2, 2011 4:44 p.m. Jamesc2123 Reader

I took a closer look at the line routing, and what happened is different than what I originally said (Edited first post for consistency).

WARNING: Crude MSPaint drawing ahead:

Proportioning valve:

So this still makes me think that the driver front/whatever rear brake that is still should have worked, since the brakes should be on two totally separate systems, to prevent this very situation.

Is is possible that either the master cylinder or the proportioning valve are damaged internally, allowing pressure to leak from one brake circuit to the other?
May 2, 2011 5:52 p.m. iceracer Dork

It's possible that the leak had allowed the master cyl. to lose all of the fluid before you noticed it.
May 2, 2011 5:56 p.m. Datsun1500 Dork

I had a front brake line pop on a 2006 car that had 200 miles on it and it resulted in no brakes whatsoever. Single circuit, dual circuit, whatever. Loss of pressure is loss of pressure.
May 2, 2011 6:28 p.m. KATYB Reader

oh you have some brakes still for 1 maybe 2 stops... but thats for a slow slow slow stop with the peddle held to the floor. yes thats how the system is. even tho yes its a split port master it happens. dont want it to happen have 2 masters thats only way.
May 2, 2011 6:44 p.m. Teh E36 M3 HalfDork

You should have pressure to two brakes. The master cylinder in my 1969 MG Midget has a segregated reservoir, so unless there is some way for fluid to transfer between the two systems, they should theoretically be separate. In theory, you shouldn't lose both ends if one fails. I'm pretty sure it acts as if you have two masters.

I only pointed out the specific car to show that the idea has been around quite a while.
May 3, 2011 7:20 a.m. foxtrapper SuperDork

The split brakes whereby you keep the other two wheels when one circuit goes is a great theory. But that's about all it is, a theory. Very rarely have I encountered a car where it works.

Note for example when you bleed the brakes. Open one wheel bleeder, and the pedal goes to the floor.
May 3, 2011 7:40 a.m. Datsun1500 Dork

Teh E36 M3 wrote:

You should have pressure to two brakes. The master cylinder in my 1969 MG Midget has a segregated reservoir, so unless there is some way for fluid to transfer between the two systems, they should theoretically be separate. In theory, you shouldn't lose both ends if one fails. I'm pretty sure it acts as if you have two masters.

I only pointed out the specific car to show that the idea has been around quite a while.

My 68 Datsun 2000 has the same setup. 1 for the front, 1 for the back. That being said when you open any bleeder the pedal goes to the floor. What happens on the MG when you open a bleeder?

That being said there is not a way for the fluid to go between the 2 reservoirs, but the mastery cylinder plunger controls both so the pressure is tied together, lose it in 1 spot, you lose it everywhere. The only way to have to separate circuits is 2 master cylinders side by side.
May 3, 2011 9:05 a.m. WilberM3 HalfDork

when my bro's m5 blew the rear hard line (5mph into the garage) it lost nearly all brakes, but a few quick stabs built enough pressure to stop it before going through the wall. i can see higher speed being a frightening experience...
May 3, 2011 10:00 a.m. dculberson Reader

If you ever drove a car with single circuit brakes after a line failure you would know what terror is. A dual circuit master will lose a lot of pressure but brakes are still minimally functional. A single circuit master losing pressure means ABSOLUTELY NO BRAKES. Like you aren't even pressing on the pedal. I lost brakes on my A100, pulled on the parking brake - the handle came off in my hand - blew through a red light narrowly missing a Cadillac, and nursed it to a stop by downshifting and curbing.

Years later, I lost a brake line in a Celebrity, dual circuit master, and while the pedal would go to the floor, the car would slow. It wasn't great or safe but it could be limped home.

So to clarify, when you said you had no brakes, do you mean absolutely nothing whatsoever? As in, pushing the pedal had absolutely no effect on speed? You had to get it towed from where it was?
May 3, 2011 10:18 a.m. Datsun1500 Dork

dculberson wrote:

So to clarify, when you said you had no brakes, do you mean absolutely nothing whatsoever? As in, pushing the pedal had absolutely no effect on speed? You had to get it towed from where it was?

This is exactly what happened on a new car. It was the day after I bought it and while coming off an exit ramp I hit the pedal and felt it "pop" then no brakes. None. It had to be towed to the nearest dealer. The front drivers side had split at the caliper.

I'm pretty sure that is the only time I have used the emergency brake as an emergency brake :)
May 3, 2011 10:37 a.m. iceracer Dork

In theory, you should have had some braking since the car does have two seperates systems albeit with one master cyl. which has two pistons. Never having experienced brake failure,I have no clue as to how it reacts.

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