mj_wils1970 New Reader
July 2, 2016 7:51 p.m.

I moved my 1990 Miata into the garage last night without a problem but when I went to move it today the brake pedal went to the floor. The hard line going to the back failed and it was flowing out pretty good.

Now this is a 25 year old car so I'm not surprised or upset, however the idea of completely losing the brakes at speed worries me. I'm going to replace all the hard lines now.

Is there an upgrade I can do to prevent a total loss of fluid? The service manual states the car has a tandem circuit, is that the same as a dual?

My son will be driving the car in the next few years and it scares me to think of him being without full braking power.

Keith Tanner MegaDork
July 2, 2016 8:42 p.m.

It has a dual circuit - the front and the rear are separate. If you lose one end, you should still be able to stop the car.

I think the factory lines are stainless, so it's pretty unusual to see this sort of failure on a Miata. Those ones on the rear subframe are probably the most affected, you can do a visual check on most of the rest.

BrokenYugo UltimaDork
July 2, 2016 8:45 p.m.

Anything 1968 on up is dual circuit, with one blown the pedal will be low, but you will have 50% brakes for several stops/hours as the fluid drips out, though it can be quite exciting when this happens in a panic stop on a cross split (rather than front/rear split, I'd guess Miata is cross split) car as it will try to rotate around the remaining front brake (easily corrected with some steering). A lot of the older stuff had dual reservoirs so even if one ran dry you still have brakes on the other, dunno why they quit doing that.

The best protection against this is to inspect every inch of the lines and hoses annually, surface rust=ok, scaly tree bark looking rust=quit driving it. An extra 5 minutes when you rotate the tires

Chas_H Reader
July 2, 2016 8:52 p.m.

Dual circuit brakes give the driver some braking ability if one circuit goes out. Speaking from experience, I can tell you it was never 50% but closer to 10 %. Miata does not use stainless brake lines. If that was the case there would be no reason for the greenish protective coating on them.

Keith Tanner MegaDork
July 2, 2016 9:42 p.m.

Stock Miatas lines don't flare like mild steel, so they're something other than what's found in the auto parts stores. They're quite a bit harder.

Some modern vehicles have a single reservoir with an internal wall, so they're effectively dual reservoirs. The NA/NB Miata system is a front/rear split as opposed to diagonal. Not sure about the later cars, but I think they're also front/rear.

The ND Miata has some interesting behavior when it gets a bubble. Somehow, the system compensates. You get a brake warning light and a ! light, and the pedal feels normal for normal braking. But get into it hard, and you'll get that double-step feel of a bad circuit and the brakes don't release for about a half second after you get out of the pedal. You can still stop, but it's quite obvious something is wrong. I found this out last weekend on the track I want to investigate it further, because it's interesting.

Chas_H Reader
July 2, 2016 10:11 p.m.

Somehow Mazda or their supplier manages to flare the tubing.

There is every chance it's not the same as found in a parts store. That hardly means it's stainless.

mj_wils1970 New Reader
July 2, 2016 10:52 p.m.

The reservoir was almost empty and the pedal went to the floor, so I'm confused as to how I should have had a percentage of my braking left.

Sorry if this is a newb question but with almost no fluid or resistance on the brake pedal, was there any braking ability left?

Not something I can really test or would want to.

I did recently install a set of braided brake lines and the FM brake brace just to add to the confusion.

Chas_H Reader
July 2, 2016 11:09 p.m.

The reservoir has a divider that should retain some fluid for the circuit not leaking. However, as you found out and I posted, the dual circuit brakes really don't preserve much braking force when one circuit is failed.

Keith Tanner MegaDork
July 2, 2016 11:12 p.m.

There are two pistons in the master cylinder. You press on the pedal, you push on one piston. The fluid in that chamber presses on the next piston. If one of the chambers is dry, there's a spring inside that you push against. Obviously, since the spring is compressible and brake fluid is not, the pedal drops. But it's enough to give you some braking power even though it doesn't feel like there's much going on.

If the reservoir is empty, there should still be fluid in one of the chambers. If you have two leaks, well, you have a problem.

As for what the Mazda brake lines are - obviously Mazda can flare them, and you bet it's with a big industrial hydraulic tool. But I have also tried to flare them, and it's very difficult with a Snap-On hand tool. Difficult enough that the failure rate is high enough that I do not recommend it. It's a very hard steel. Given the fact that they are extremely rust resistant in my experience, I hypothesized they are stainless. Whatever. Maybe they're made of uncooked spaghetti. They don't rust often.

BrokenYugo UltimaDork
July 2, 2016 11:14 p.m.

In reply to mj_wils1970:

You have to really bury it in the carpet, it will stop if all the fluid hasn't run out. The empty reservoir makes me think it started dripping last night and blew out today or something to that effect, it usually takes like an hour for the fluid to drip out, or like 8 pedal strokes.

outasite Reader
July 2, 2016 11:32 p.m.

Front brakes perform a larger percentage of braking because of weight transfer. Therefore the rear brakes are smaller and will do less braking at speed if the front hydraulic circuit fails. Most FWD cars have a diagonal hydraulic system. RWD cars use the front/rear hydraulic system. A dual hydraulic system allows the car to have one good hydraulic system if the other fails. The pedal will move closer to the floor and naturally with air/loss of fluid from leaking system it will feel spongy. Many times it will feel like the car has no brakes, but if you push further, reduced braking should occur. However, if the other system is also compromised (defective m/cylinder, air in the lines, frozen/rusted caliper pistons the back up system will not brake as designed. Since you replaced lines, is it possible air was still in one or both hydraulic systems? Also, when I installed braided lines on my NA, I had trouble getting the line going into the rear junction block to seal. It had a slow leak and as Keith has mentioned, the lines are very hard. I used a flaring tool to improve the flare and this solved the leak.

codrus SuperDork
July 3, 2016 4:05 a.m.
Keith Tanner wrote: As for what the Mazda brake lines are - obviously Mazda can flare them, and you bet it's with a big industrial hydraulic tool. But I have also tried to flare them, and it's very difficult with a Snap-On hand tool. Difficult enough that the failure rate is high enough that I do not recommend it. It's a very hard steel. Given the fact that they are extremely rust resistant in my experience, I hypothesized they are stainless. Whatever. Maybe they're made of uncooked spaghetti. They don't rust often.

FWIW, I re-flared a couple of Mazda stock lines while swapping ABS into my car, and while they're definitely harder than the copper/nickel lines I was using to put the 929 master, it was very doable with my Bluepoint hand flaring tool.

Maybe the NA lines are different than the NB ones? Dunno.

Keith Tanner MegaDork
July 3, 2016 10:36 a.m.

Difficult != impossible. It's doable, but the failure rate is higher. Not failure rate in service, but failure rate in terms of getting a bad flare.

Knurled MegaDork
July 3, 2016 11:22 a.m.

I wouldn't think that stainless is an acceptable material for brake lines. It might very well be but I wouldn't have thought it.

At any rate, I get many customers who say the car has "no brakes" when it still has a single stage remaining. Some people think that if the pedal isn't higher than the accelerator then there is nothing there, like pedal travel is what slows the car down and not pedal pressure. When one stage blows out, you WILL still be able to slow down, but the pedal will significantly be lower.

This offer not valid if you have 13 floor mats in the car, half of them folded up against the firewall.

mj_wils1970 New Reader
July 3, 2016 7:18 p.m.

I'm glad for all the sensible and thoughtful feedback for this topic. This forum truly is a great one.


outasite Reader
July 3, 2016 7:35 p.m.

Did you isolate the cause of the soft pedal and what was it?

mj_wils1970 New Reader
July 3, 2016 8:35 p.m.

I haven't been able to look at the car all weekend so no. I need to work on it in a barn, so I will be moving it and will update the thread tomorrow with what I find.

outasite Reader
July 3, 2016 9:16 p.m.

Check the m/cylinder reservoir to determine which system is loosing fluid. If it is the rear system, I would check that rear brake junction block first.

Toebra Reader
July 3, 2016 10:39 p.m.

Wouldn't stainless steel hard lines on your brakes tend to work harden and get brittle?

mj_wils1970 New Reader
July 4, 2016 12:33 p.m.

An update.

I moved the car after filling up the reservoir and the pedal moved down to the floor, pumping it just shot fluid out the hole that was 18 in front of the rear junction. Pushing down on the brake pedal didn't seem to slow the car at all so I had to use the e-brake to stop the car (there's no floor mat in the car at the moment fwiw).

In the barn I looked and there was a small amount of fluid left in the front portion of the reservoir and the back was empty.

So the front brakes should have some fluid to work on, maybe at speed they would have an effect?

I removed the line and it actually was so brittle it snapped in 2 places, I'm really glad this didn't happen on the road.

Long story short I felt I didn't have enough room to work under the car properly even with my jack stands at the highest point. Age has taken it's toll on me and I don't move as well as I used to.

I had the car towed to my usual garage that works on stuff I don't feel comfortable with to replace the line.

Once again thanks for all the useful feedback.

Knurled MegaDork
July 4, 2016 6:58 p.m.

Brake lines are weird, they can hold up to full braking pressure no problem but still be rusty enough to snap like twigs if you disturb them.

10-12 years ago you could buy OE lines from Mazda for an NA. I don't know if you still can do that.

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