Brakes holding / grabbing pt2

christojen · 05-03-2009, 09:22 PM

Hi guys I have a 1977 911 targa that i have been doing some investigative research on that i would appreciate your advice on:

The problem
The brakes intermittently, although especially when really stood on, 'hold' on for anywhere between 2 minutes and 1/2 hour. One time it happened i jacked the front up and noticed BOTH front wheels were locked. By locked i mean you can still drive but it is obvious the brakes are on, like driving with the handbrake on. I jacked up one back wheel and it seemed pretty free.

The solution

I thought i had it nailed, i changed the flexible brake lines to all four wheels. This has done little to help it. After doing this, whilst driving one time it happened and the brakes were locked on, and i pulled the pedal back manually and i fixed the problem! Unfortunately this did not happen on subsequent attempts to fix the problem in this way.
The next thing i tried was to disconnect the vacuum to the booster, which appears to have reduced the duration which the brakes lock on. I dont know how reliable this is but it 'seems' to have helped. I should note that i can repeat the problem to some degree when the car is off and stopped.

The Answer
So i am wondering what you all think might be the problem...
I am suspicious of the master cylinder piston getting stuck in, but i dont really know how it works. I am also suspicious of the brake calipers, but it seems unlikely that they lock and release together, which seems to be the case.

Let me know what you all think or tests to try. My mechanic suggested opening a bleeder valve when it is locked to see if fluid comes out. Other than that i am replacing things piecemeal...

Chris in Aus (i should mention it sat for 7 years before i got it.....)

rajah911 · 05-03-2009, 10:49 PM

Hi Chris,

I suspect there may be some corrosion on your master cylinder bore/piston which is causing it to stick intermittently as you have described. Brake fluid is 'hygroscopic' which means it actually attracts and absorbs moisture over time (eg. 7 years) - hence the danger of rust forming. My first course of action would be to remove the M/Cyl, dissassemble it, inspect and rekit/replace (depending on condition).
Regards,
Steve

christojen · 05-03-2009, 11:06 PM

Thanks Steve,

Sounds like the most likely problem.

I have a few questions:

Does this make sense that only the front wheels appear to be locking? I know there is a line for each front back - is the front first in line such that it would lock without the back locking?

Is this sort of thing ever likely to "sort itself out" (free up with use) or am i dreaming?

How hard is the disassembly and inspection / rebuild of a MC?

TIA

Chris in Aus

Bill Verburg · Bill Verburg's Garage

I would suspect the m/c too. There is a relief port that allows fluid to return to the reservoir. If this gets plugged for whatever reason the breaks can't release.

69911e · 05-04-2009, 03:35 AM

If the MC checks out, look at the pedal return spring, verify no pedal assembly binding, and verify rod length between MC and pedal.

christojen · 05-04-2009, 03:41 PM

any other ideas out there?

rajah911 · 05-04-2009, 09:23 PM

Chris,

Yes, there is a seperate line from the m/cyl for each of the front and rear brake circuits. Now, I'm not sure about the '77 models, but according to the w/shop manual for my '86 Carrera there is a 'pressure regulator valve' fitted to the rear brake circuit, the purpose of which is ..."To prevent rear wheel lockup and loss of steering control during hard braking, hydraulic pressure to the rear calipers is reduced by a rear brake pressure regulator". If your car has one, it should be located in the rear brake line from the m/cyl, in the luggage compartment under a small cover.
If you do find one, then maybe it is actually doing its job, and this is why your rear brakes are not locking on like the front? Just a stab in the dark...

Cheers,
Steve

Flieger · Flieger's Garage

The "knee" type pressure regulator was only for the Carreras with their over-sized rear pistons, which only started with the wider 24mm vs. 20mm rotor thickness and the front A as opposed to M caliper. 1977 911 have no pressure regulator.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by christojen

any other ideas out there?

It's easy enough to exercise the caliper pistons, just pull the calipers and check for full movement of the pistons by hand

The m/c has 2 inline equal sized pistons, in this case 19.05mm

one seves the front circuit(both front wheels) and the other serves the rear circuit(both rear wheels)

here is what it looks like internally

christojen · 05-05-2009, 03:12 PM

Thanks for the illustration Bill, can you or anyone else tell me if it is possible to dissassemble, clean and rebuild the master cylinder?

I am of reasonable technical ability, but if it is easier i think the better option is to get a new replacement, my mechanic tells me they run about $250 Aus. I guess i should check pelican for them too but they are a bit bulky to send to aus.

Any comments from people who have rebuilt the M/C would be appreciated.

Chris in Aus

christojen · 05-05-2009, 03:17 PM

Bill, your illustration also looks more like a 65-77 model M/c whic resides behind the pedal.

I have the vacuum assisted type which resides in teh front luggage compartment, and is operated by a pushrod.

I presume they are still similar in their operation?

Chris

Bill Verburg · Bill Verburg's Garage

My bad, I did post the pre'77 non boosted 19.05mm m/c

Your 20.6mm boosted is similar internally.

AFAIK there are no repair kits fot those m/c, Porsche stopped supplting them for liability reasons

New ATE replacements for yours from Pelican are ~$200 and weight ~3lbs