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Question Master Cylinder failure symptoms...

Hi,
Does anyone know what is the sympton of a failing brake master cylinder?

Does it translate to longer or softer pedal travel? Or will my brake actually fail?

And finally how could I confirm its failing / failed?

thanks...
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Old 05-29-2003, 08:35 AM
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Soft pedal that needs to be pumped up is a start. But, calipers that have the pistons returning too far into the bore can also cause this (usually when new pads or piston seals have been installed). Noah's constant pressure test is the best place to start. If the pedal refuses to stop traveling, then you've got an external leak in the system, or the MC is failing.

Monitor your fluid level in the reservoir. If you're losing fluid, it's not necessarily the MC. Leaking can occur at caliper or piston seal, bleed screw, cracked hard line, old and tired flexible line, brake warning light switch, ....... a number of things.

If the fluid level stays constant, the brake fluid can be sneaking past the seals of the MC and causing a loss of pedal pressure. This condition doesn't show up as a leak because the fluid getting past the seals stays within the system and doesn't leak out.

Or you could have a swelled rubber flex line that's causing the soft pedal. It's been said that the lines can get old and pliable and swell when you press the pedal. I've never seen this myself, but others have mentioned this occurrence.
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Old 05-29-2003, 09:33 AM
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Patrick,

All of the above are good checks for a failing M/C, but a failed circuit will give a dramatic additional free-play zone in pedal travel ... then firm up as you reach the bottom of travel for the remaining good circuit. The circuit that fails is almost always due to worn cups & seal. Usually there is pitting on the bottom side of the master cylinder bore due to rust/corrosion from old brake fluid.
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Old 05-29-2003, 09:48 AM
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I am still narrowing down the exact behaviour but it manifest itself only when I brake hard on the track; including the 1st ever time so that kinda implied its not really temp related (the pedal also does not sink deeper after repeated use).

Have just installed SS brake lines & rotors. Still lots of pads left and I'm not lossing fluids. It just that the pedal is very spongy & travel is long (compared to what I remembered last fall).

So it sounds like leaking around MC seals is the closest "symptom"? Is there any way I can confirm that without simply dropping in a new one?

And thanks for all the good advice here !
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Old 05-29-2003, 10:25 AM
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Patrick,

The only way I know of is to disassemble and clean all of the master cylinder internal parts ...

The newest clue you provided is the new stainless flex hoses ... I have experienced sponginess due to slight swelling of the Teflon liner ... when extremely high pedal pressure is applied to static car on stands, and the lines could be visually monitored. Might be worth a look if you can get the car up on stands and an assistant can be rounded up to mash on the brake pedal for you ...
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Old 05-29-2003, 10:42 AM
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Patrick, I am experiencing the same problem. It's not a remarkable change but I do loose pedal slightly( a bit too much for comfort). I will be installing a new MC this weekend. I have ruled out the other components since I replaced the front calipers this spring. I wil keep you posed as to the outcome.

Cheers, James
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Old 05-29-2003, 10:57 AM
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Mine just failed, and I replaced it a few months ago. First of all, the brake pad light goes on. Mine did, and I thought the system was busted. It wasn't, it just took a few more months for the master cylinder to get bad enough I noticed it.
The symptom was, the brake pedal continued to go to the floor while you held it firm.
Common occurence for me was, I'd stop at a light on a slight incline with my foot on the brake, and at first it took 10 or 15 seconds before the car would start to roll, but near the end, it would start to roll in just a second or two. You could also keep pressure on by pushing the pedal as it slumped away however, you'd eventually end up almost at the floor.
A symptom for me was not pumping. It always reacted well for a sudden stop, it was just the seepage.
I replaced it, and it's all good as new.
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Old 05-29-2003, 11:51 AM
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If you just installed new lines, then you may need to bleed the system a few times. One time does not always do the job.
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Old 05-29-2003, 01:31 PM
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