[Goldenblack440]

Can anyone clued up on brakes tell me what they think might be the problem here, with my brakes. Boosted front ventilated Disk. 10" drum, all stock except for new aluminium master cylinder with slightly bigger piston bore. Brakes work quite well on initial application, that is, they will easily lock the front up with a hard jab (255 front tyres), but if i hold the brakes on in a hardish stop they fade to nothing, almost as if the booster has given up. It feels as if there is not enough vacuum, but although the engine has a long duration cam it still makes about 10" at idle and 12-14 cruise. It is most noticeable at very low speeds, just above idle, when a second and third application results in brakes going to almost nothing.

The other problem happens when braking from reverse. Almost nothing! Need massive pressure to stop from a crawl in reverse. And there is a terrible groan/shudder from the front brakes at first as well. I thought the rear drums were supposed to work better in reverse?

I have checked everything, proportioning valve works, no leaks from slave cylinders, all bled well, the back brakes work with the car on jack stands and the wheels spinning. But on jacks and in REVERSE, even with no load, it is hard to get the wheels to stop. Handbrake works well. Plenty of shoe pad and front disc pad there, rotor well in thickness limits.

Any ideas please? My guess is either a vacuum leak at the booster and therefore is most noticeable at low engine speeds with this camshaft or the engine is just not producing enough vacuum for the booster to work the way it should? But that wouldn't explain why very little brakes in reverse?

Thanks

[Jon]

[

It is most noticeable at very low speeds, just above idle, when a second and third application results in brakes going to almost nothing.

Only time that has happened on mine is when it is about to die. ( low vacuum) Possibly a leak in the booster or valve in this situation.

[Goldenblack440]

Thanks, Yeah i reckon it could be a bit of a leak in the booster as well, but could be a combination of things.

[ianandjess]

hey steve if the vacumn is low the pedal will be very heavy as when the motor is not runnig
is that how it feels
cheers ian