Brake Failure

[snowshoe]

In theory, car brakes should not totally fail if you spring a leak. However, can anyone explain how the pedal reacts and will the good circuit keep working and for how long? My experience was that the pedal went totally to the floor without resistance (fortunately I was going uphill and slowly at the time.)

 

[GregLocock]

It depends where the leak is, and it depends on the exact setup of your brake circuit.

"However, can anyone explain how the pedal reacts and will the good circuit keep working and for how long?"

Roughly speaking the pedal should go half way to the floor, from its normal engagment point, and the required pressure for a given decleration should increase somewhat. The ultimate braking performance of the vehicle will almost certainly be reduced. For a European passenger car the requirement is, very roughly, 60% of the braking performance for 70% of the pedal effort.

The good circuit should keep working indefinitely, so long as the fluid level is kept topped up - this may not happen as the leaky circuit may keep taking all the spare fluid.

There are quite a few single points of failure in a brake system, and the extra stress imposed by a failure in one circuit could cause the other to fail, if it is an age or fatigue related problem.

For instance, if one piston is removed entirely from a front brake in a system that uses two 2F+1R circuits then all the fluid will end up on the road and you will have NO braking, even with this sophisticated split.



Cheers

Greg Locock

 

[Tmoose]

".... The good circuit should keep working indefinitely, so long as the fluid level is kept topped up - this may not happen as the leaky circuit may keep taking all the spare fluid."

I think all my cars have a divided master cylinder, with one chamber for each system.

 

[GregLocock]

But I bet you only have one reservoir. This is divided at the bottom to stop the leaky circuit from taking all the fluid, but its function presupposes that the remaining circuit is in perfect health.

Cheers

Greg Locock

 

[Tmoose]

"I bet you only have one reservoir."

Well, the old Volvo has one reservoir and one cap, with a dam that keeps one circuit under its own 1/2 inch of fluid.

My Ford truck masterC looks like this................

http://riversiderebels.com/amccf/tech/new/brakes_master_cyl_clip-on.jpg

Its divided right to the cover gasket surface, and the gasket has twin accordions, and the cover has dual bumps.

What did I win?


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[76GMC1500]

Have you disected or repaired the brake system yet? I would think a failure like that would be in the master cylinder or maybe you broke your linkage.

The brake system on my truck is much like that of the Ford shown, I've got 2 seperate reservoirs, 2 separate lines, and the only link between the 2 brake systems is that the lines both go to a differential pressure switch (which tells you when your system fails) before before the lines run to the calipers.

 

[snowshoe]

The car is a '94 Mazda 626. We had it towed to get repaired (just prior, I started the engine and the pedal still went effortlessly to the floor. I did not attempt to move it, so don't know if there was any braking still available). The mechanics at the dealer found a leak in the rear brake line caused by age and corrosion. They replaced both rear lines (the other, being just as old was likely to suffer a leak soon otherwise).
My question to them at the time was why the pedal felt like it did, because with the pedal hitting the floor, how could the brakes be working? There answer was to explain the theory of how brkes are supposed to react and that I should have had some braking still available.
I suppose it would have been better if I could keep going and find out, but the risk involved detered me. The brakes have been fine since the repair (3 + months) so that eliminates any other component failure.

 

[JayMaechtlen]

The pedal in a tandem-master cylinder should not go to the floor if either one of the circuits fail. I would be very suspicious of the master cylinder and of the front brakes.
(the pedal will normally go farther down when only one circuit is working, but should not reach the floor)
I assume that a brake warning light illuminated when the brakes failed? Many systems have proportioning valves or the like, which can also fail?
regards

Jay Maechtlen

 

[zedrider]

I recently suffered a brake failure on a 1989 Ford Fiesta (FWD, 1.1L, sub-compact, hatchback for all you Merkuns).

The pedal went right down to the floor*, but retained marginal braking power - enough to get home.
No warning light came on (low brake fluid warning only) and I strongly suspect slave cylinder seal leakage, since the dust boot had a smear of fluid in it.

I will know for certain tomorrow when I get it back from the local garage.

*As far as I could tell, while slowing from 70mph to about 10mph down a 1in20 motorway slip-road. I was concentrating more on not getting T-boned at the junction, which was approaching rather more rapidly than anticipated.