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Brakes Won't Hold Pressure.

Snowman
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Snowman Grand Master

Snowman

Posted
Posted

Ok - I have a brake issue I'm not 100% sure how to fix or what's causing it.

Basically I'm not holding brake pressure - aka the pedal is going to the floor.

If you "pump" the pedal and hold it - it will hold pressure fine however. But if you release the pedal about and wait 15s or so the pedal will go straight back down to the floor again. I've bled through about 2L of brake fluid through and the problem is still there. And no stage have we seen any signs of bubbles when bleeding either.

The problem has been there ever since I experimented with a hydraulic handbrake system. When you applied the hydraulic handbrake you could feel the pressure come back through the foot brake pedal however. The hydraulic handbrake has since been removed as it was usless for my wanted applications.

Is it possible that the hydraulic system has wrecked my master cylinder or is there something else I should be looking at? (When installing the hydralic handbrake we did accidently run the system dry once (no fluid) and apply the foot brake). Or is it possible the problem is somehow related to the ABS unit instead as the ABS and the hydraulic handbrake were not friends?

Any advice is appreciated as I don't want to go pulling things appart unnecesarily.

Cheers.

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Mr. Oizo Grand Master

Mr. Oizo

Posted
Posted

If you push the pedal all the way down while bleeding, you've probably ruined the seals in the master cylinder.

When you are braking in your car, your brake pedal usually only goes about half way down so the piston in the master cylinder only goes part way of its full travel. If the master cyliner is old or the fluid has not been flushed regularly, the far end of the master cylinder can get pitted from rust or have grit in the fluid down the far end. If when bleeding, you push the pedal all the way down, you can cause damage to the cup seal in the master by running it over the rough surface down that end.

If the master has been freshly rebuilt and the bore given a light hone, then it shouldn't be a problem.

GTRgeoff Grand Master

GTRgeoff

Posted
Posted

Snowy, sounds like the master cylinder needs a freshen up mate.

Just in case it isn't, is it losing any fluid? Any leaking inside the footwell? Around the ABS unit? Does it do it with the ignition on and off?

R32's have a real problem with leaking ABS units so not unheard of.

Anyway sounds like the classic master cylinder seal failure.

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