Brakes not working WHY?
Discussion
Changed a leaking piston yesterday, bled the brakes, but the brake pedal looses pressure when you keep your foot on it, and you need to "pump up" the brakes to get any pressure in the system.
Would this mean there is still air in the system, or may the master cylinder need changing. Ta for any info.
Would this mean there is still air in the system, or may the master cylinder need changing. Ta for any info.
If the pedal sinks under sustained pressure either the master or slave are leaking. A leak at the slave will result in external fluid loss. A leak at the master won't. It's easy to damage the m/c seals while you are bleeding the brakes, which I assume you did after your work on the system.
If the brakes hold pressure once you have pumped them up, then yes, there is still air in the system and you haven't bled them properly.
If they pump up but then the pedal slowly sinks under your foot, then there is a leak or a damaged seal somewhere - particularly if you are losing noticeable quantities of fluid and are having to top up the master cylinder resevoir repeatedly. As GreenV8S says, though, a damaged wheel cylinder/caliper piston seal will leak and lose fluid but a master cylinder won't, necessarily. There are other places for leaks to occur apart from the piston seals, though - you might have a damaged brake line or a bleed nipple that's not tightened or seated correctly.
Seals often go when they have been moved beyond their recent working range, because they suddenly find themselves running in a corroded (abrasive) part of the bore.
Having said all that, I'd still say there's something like a 80% probability that it's nothing more than air in the system and you just haven't managed to bleed them effectively, yet, though!
If they pump up but then the pedal slowly sinks under your foot, then there is a leak or a damaged seal somewhere - particularly if you are losing noticeable quantities of fluid and are having to top up the master cylinder resevoir repeatedly. As GreenV8S says, though, a damaged wheel cylinder/caliper piston seal will leak and lose fluid but a master cylinder won't, necessarily. There are other places for leaks to occur apart from the piston seals, though - you might have a damaged brake line or a bleed nipple that's not tightened or seated correctly.
Seals often go when they have been moved beyond their recent working range, because they suddenly find themselves running in a corroded (abrasive) part of the bore.
Having said all that, I'd still say there's something like a 80% probability that it's nothing more than air in the system and you just haven't managed to bleed them effectively, yet, though!
This may be way off, but then again - leak at master, on seals toward the servo , is not common, and sometimes not easily spotted.The one i had resulted in slow loss of fluid. However easy way to spot this is to hit brakes hard ( better when stopped ) and look for white smoke out of exhaust.Brake fluid burns white.
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