Brake Issue - Thoughts please

[Jimmy]

I have just rebuilt all calipers as 3/4 of them had corrosion on the piston. They were completely taken apart, cleaned up, regreased and reassembled. I bled the brakes removing all air and old fluid in the system as you would do normally. While pumping the pedal, it became rock solid as per normal. Upon finishing the last caliper, the brake pedal felt nice and firm as you would expect from a full brake rebuild and bleed. Here's my problem:

Moments after starting the car and driving down the drive way, the pedal instantly became squishy, much like if there was air in the system. I was a little put off by this so I jacked the car up and bled all the brakes again (RL, RR, FL, FR) and there was NO air in the system at all None of the calipers were leaking either. I started her up and the same damn thing happened; the pedal felt nice a firm to start with and then instantly changed to squishy. I went for a slow drive and jumped on the brakes a few times. The rear left would always lock prematurely and I couldn't get any of the other wheels to lock at all.

Given that there was no air in the system the first time I bled it I consider it unlikely that a caliper or the lines are leaking. I had noticed the braking capabilities declining a fair bit hence the rebuild. Could this be a brake booster issue?

Another thing I noticed while pumping the brake while stationary and with the engine running was that the revs would increase ~300rpm every time I pumped the pedal. Is this normal?

So I guess I have three problems:
1) Loosing pressure in the pedal with no leaks or air getting into the system,
2) Rear left locks first and prematurely while none of the other wheels will lock and,
3) Revs jump ~300rpm while in neutral and pumping brakes (this could be normal but I haven't noticed it before)

If worse comes to worst, I'll take it to ACT Brakes/Carco and get them to go over it.

Your help is much appreciated. Thanks in advance,
Jimmy

[AJ]

next step is the master cylinder mate, once they start to degrade, fluid can leak back past the piston losing you pedal feel, pedal firmness & causing the wheel furthest from the master cylinder to lock up infrequently (LR).........my advice is DON'T rebuild a master, get a new one, rebuilds never last long.

my 2c

[rvlovell]

The fact that one wheel locks first would be something to do with your brake lines. I would have thought still some air or possibly crap in the lines. Possible a sticking piston

[project.r.racing]

next step is the master cylinder mate, once they start to degrade, fluid can leak back past the piston losing you pedal feel, pedal firmness & causing the wheel furthest from the master cylinder to lock up infrequently (LR).........my advice is DON'T rebuild a master, get a new one, rebuilds never last long.

my 2c

x2 - i'd be popping ya head under the bonnet and looking for a leak near it also.

[yoda7356]

As AJ says check the master cylinder. There is also a possibility that the master cylinder is leaking into the booster. This leak could be part of the problems with the rise in revs when you pump the brakes. Hope this helps some.
John(yoda).

[PT]

next step is the master cylinder mate, once they start to degrade, fluid can leak back past the piston losing you pedal feel, pedal firmness & causing the wheel furthest from the master cylinder to lock up infrequently (LR).........my advice is DON'T rebuild a master, get a new one, rebuilds never last long.

my 2c

Interesting experience - in my experience the rebuilds have lasted longer than the new ones - as generally they are rebuilt with better quality components. Most aftermarket master cylinders are imported from China these days, and I have to question their quality.

I guess it all comes down who does the work.

[Matty]

Does this happen as soon as you start the engine? If so, that's just the vacuum booster kicking in.

[Jimmy]

Thanks for the advice chaps, it has helped although I am still trying to diagnose the problem.

Does this happen as soon as you start the engine? If so, that's just the vacuum booster kicking in.

Yes. When the engine is off, the brake seems to be 'normal'. When the engine is switched on, the brake pedal drops, looses all feel and the revs rise. I've had a look at the master and it doesn't appear to be leaking. If it isn't the master that's the problem, then what is it, why is it doing it and what can I do to fix it? (I like to understand processes rather than just doing them)

Could the master still be the problem even though visually there doesn't appear to be any leaks on the outside?

If it is the master, I will have for a rebuild as PBR are out of stock Australia wide for the next two months. An oem replacement is in the vicinity of $700+. Although brakes and tyres are not things I like to skimp on, $700 is a little out of my budget at the moment.

Jimmy.

[Mr Morlock]

You have described the symptoms and you know what you have done. I would be heading to the brake specialist to get it looked at. You also cleaned up the pistons and it might have been an idea to have shown these to the brake guys before re installing. You will not need to spend $700 on a new master cylinder- there are other options and kits surely are procurable in under 2 mths.