Rathbo

I have been rebuilding a 1984 944 NA after a total loss three years ago. I have transplanted virtually 100% of the components and systems from the 1984 shell into a new shell and in the process have either replaced or had rebuilt everything. However I have been having a problem resolving a soft brake issue. I have replaced the Master Cylinder as well as the Master and Slave Clutch cylinders, all of the brake lines, I have rebuilt all forur calipers with new seals, springs, retaining pins and clips and I have properly bled all of the new lines with a a Motive Power Bleeder and have good fluid flow. However I still have a very soft pedal and diminished stopping.
So is it possible that I have mismatched parts?
This is what I have:
A Brake Master Cylinder with the numbers H 05 038/3
A Brake Boster with the numbers 944 355 02 503
A Master Clutch Cylinder with the numbers 0850
A Slave Clutch Cylinder with the numbers 10792
Any help with this matter is appreciated

xsboost90

do you have good vac to the booster?

Rathbo

Good question. But how do I determine sufficient vacuume?

xsbank

Did you install the calipers with the bleeders up?

944Ross

Did you bench bleed the M/C before install? Even with a pressure bleeder, there are several places where air can hang out in the system. The design of late-models has the lines out of the M/C going up, over, then down in the tunnel area to the rear brakes, makes a perfect air trap.

944Newb

From my knowledge 17-22" hg

Rathbo

Calipers were installed with the bleeders at the top.
I did not bench bleed the Master Cylinder. Do you think it might be a useful exercise to disconnect the brake line at the junction point directly in front of the battery and bleeding from that point?

944Ross

If the M/C wasn't bench bled, that's the first thing I'd do. Major PITA but if it isn't air-free, anything else will be a waste of time.

Rathbo

Yo 944 Ross do you think a modified version of bench bleeding would work as well as complete removal of the master cylinder. As in: 1. Disconnect hydraulic lines exiting the master cylinder. 2. Insert appropriate sized stop/plug into where the hydraulic lines were connected. 3. Individual in driver's seat very slightly pumps brake pedal. 4. Observer observes reservoir on top of master cylinder to watch for any air bubbles as pumper pumps.
I am just trying to avoid taking the master cylinder out of the vehicle right now.

944Ross

Yeah, that is still what's considered "bench bleeding", the MC is not hooked up to the system and it recycles fluid. In fact I think that's the way most people bleed them. The only advantage to doing it in a vise or whatever is that you can tip the outlet end up so it speeds up the process.

UltimatE157

Insufficient vac to the booster would create a harder pedal, even with air still in the lines. A false sense of good brakes. When running, it would still be as hard as with the engine off.

Sounds to me like you need a bench bleed on the master, as Ross has suggested.

Rathbo

Thank you all for your advice. I will give that a shot tomorrow and let you know of the results.

Rathbo

I have continued to struggle with these brakes. They remain soft as in a mushy pedal. So I purchased a Motive Power Bleeder (which is an AWESOME investment!) and I rebled the brakes and clutch slave cylinder. Resulting in SAME CONDITION SOFT BRAKE PEDAL. So I bench bled the master cylinder in this manner. Disconnected all lines from MC and inserted appropriate plugs in MC. Attach Motive Bleeder to brake fluid reservoir. Slightly pumped brake pedal and observed small bubbles leaving reservoir and flow out through clear bleeder tube on Motive Bleeder. Continued pumping action until no evidence of air bubbles. At this point the brake pedal is rock hard. Reconnected each line to MC. I performed the same procedure at the point where the brake line connects at the engine wall just in front of the battery. Resulting in a rock hard brake pedal. Then I rebled each brake line at the caliper and checked the brake pedal each time. Resulting in a rock hard pedal. Next step was to turn the engine on and try the brakes out in the driveway and there I am again with soft mushy brakes again. When I turn the engine on and depress the brake pedal and they are mushy I can hear air from the brake pedal. So I am gonna take a look at all of the vacuume lines. Any other suggestions?

jeffrsmith

If you are hearing air inside the car when you depress the brake pedal I would suspect the booster. Did you replace the booster with a new or second-hand unit? Even if you replaced it with a new unit it could still be bad.

Rathbo

It was a used booster. How would I go about testing it? How is the booster supposed to be sealed between the mounting face of the booster and the engine firewall?