Problem With Rear Brakes

[AirCooledExcellence]

Hi All,

I've recently (250 miles ago) overhauled my rear brakes, rebuilt the calipers, new hard & soft lines both sides, new rotors & pads.

I've bled and re-bled serveral times and it seems that under heavy breaking the fronts will lock up quickly, yet the rears don't.
After a spirited drive the front rotors are scorching hot (as you'd expect) yet the rears are just warm.

I've visually checked the gap between the pad and rotor and can't see daylight so that seems OK. I've also checked for leaks and found none.

Any other suggestions? could my master cylinder be faulty?

Cheers

[Early_S_Man]

Tim,

It does sound like your master cylinder is the problem. Has your rear fluid level dropped any since the repairs were completed? If so, then disassembly of the master cylinder will probably reveal pitting and rust has partly detroyed the cups and seals for the rear circuit, and rhe missing fluid was sucked into the ehgine by the booster vacuum.

A new master cylinder will probably restore your braking efficiency.

[AirCooledExcellence]

Warren,

The fluid level has not dropped at all, would I definitely have fluid loss with a faulty cylinder?

Cheers

[Early_S_Man]

Tim,

I would normally think so, but with only 250 miles, it might be a negligible amount. Has the mileage been mostly highway driving, by any chance?

[AirCooledExcellence]

Warren,

The majority of my driving is around town, hardly any highway.

Cheers

[Amac]

Tim,
I have the same problem!
I've never been able to lock up
the rear brakes.
I know that the lighter front would
lock up first, but my rear never locks.
Can anyone tell us if there are valving
tricks to get more pressure to the rear?
Part of my problem may be tire size,
315/35/17"

[Early_S_Man]

Tim,

I suggest a test ...

Get a 3/4" drive socket and breaker bar for the axle nut and put a 5 - 6 foot pipe on it ... jack one wheel off the ground and secure the car with a jack stand. Have an assistant start the engine and stand on the brake pedal while you try to turn the axle with the breaker bar & pipe ... if you can turn the axle clockwise against the brakes, they aren't working properly in the rear circuit!

[Jim Smolka]

One some 911s, (84-89 Carreras for sure) had a proportioning valve to prevent the rear from locking up.

[slakjaw]

what were the brakes like before the overhaul?
have the fronts been rebuilt?

i would think a caliper(s) thats not retracting properly would cause this kind of behaveior.

but more than likely its gonna be your master cylinder.

did you run the rear res dry?
if so you may want to try and bench bleed your current MC before spending alot of money.

Kyle
71 911

[makaio]

I'm with Warren on this.

My brakes had never been up to what I consider Porsche grade, untill recently when I changed the master cylinder. It wasn't leaking much at all, just barely. I couldn't even tell untill I pulled the booster assembly out and could see the ever so slight weaping. It made a huge difference. Now my brakes feel much more solid, and I can lock them up the wheels at will.

Prior to this, I had replaced the front calipers, re-built the rears, new SS lines, new pads, and plenty of flushing with ATE Super Blue. They worked good, but nothing like they do now.

And if you do decide to replace the master cylinder, I wouldn't waste the time, or make a mess trying to bench bleed it. I just bled mine with 2 liters of Super Blue, and my Motive Power Bleeder, and it worked fine.

[AirCooledExcellence]

Thanks Guys,

looks like a new master cylinder is in order then....

Cheers

[Roy M]

Tim

Don't know if this will work but worth a try before spending any money!

Whilst you say there's no daylight between pads and rotor try removing the pads and replacing with thin (6mm) hardwood, press the pedal a few times and then remove the hardwood and refit the pads. This made an amazing difference to the fronts of mine (I got the tip from this board) so may work on the back (my logic is that if its the same sort of problem then the fronts are locking up before the backs actually grip the disc)

I may be totally wrong and it may make no diffrence at all but it's quick and cheap! (a bit like me really)

[AirCooledExcellence]

Thanks Roy,

I'll give that a go, although I'm now convinced it's the master cylinder.

My brakes have never felt good enough and I've replaced/rebuilt everything other than the master cylinder.

Cheers