Brake Master Cylinder Issue? Carrera 1988 911

[cmvasquez]

Hey guys,

I have a 1988 911 Carrera and I am in the process of rebuilding the braking system. I did a Caliper overhaul and installed new stainless brake lines. I went to bleed the system and I could not get any pressure to build up. I used the family and friends bleeding method and pumped a good gallon or two of brake fluid through the system and no luck. The problem is that with the system closed a slight amount of pressure is able to be build up, just enough to push the pistons flush on the rotor but after like 5 seconds or so the pressure is gone and the system must be pumped again. I checked the system for leaks and there are none.

What am I doing wrong?

Did I maybe burst a seal in bleeding the brakes and now the master cylinder is not creating pressure?

New Brake Booster? (Please NO!!!)

Also the Car was sitting for a long time before the system was bled. Could it be that so much air got into the master cylinder that there was no way of creating the initial pressure necessary to operate the system?

Should I maybe use a pressure bleeder to fill up the master cylinder a bit more or is that not going to change anything?

As always, Thanks for your time guys.

CMV

[ganun]

When you bleed by pushing the brake pedal to the floor, the seal inside travels further than normal and gets damaged by corrosion build up inside. A new mc is the fix.

[osvolant]

I have a similar problem in my 88 Carrera and also need help. I rebuilt all 4 calipers (one was leaking after a DE) with ATE parts, which took me a while. When I refilled the system and bled it, using a Motive bleeder, the pedal was nice and hard as long as I had the pressure ( ~10 psi) on the MC reservoir (yes, I pinched off the vent line). Once pressure was released, the pedal went soft again. This is with no vacuum boost. Phenomenon continued after multiple rounds of both pressure and conventional, 2-person bleeding.

I finally gave up and took it to a Porsche shop, who re-bled everthing using vacuum, pressure, incantations... Anyway, he made it a little better and the car stops ok, it just doesn't have that firm pedal feel. He checked for knockback, leaks, etc. I noticed I can push the pedal almost to the floor with steady pressure and engine off.

So is my master cylinder bad?

How hard is it to replace the MC? Shop wants to charge 4 hours labor for that. Thanks to all; sorry if this has been covered before but I did search the forums.

[rekstein]

yes you need a new master cylinder same problem upgrade to 19mm master problem solved

[Walt Fricke]

MC replacement on your car ought to be easy compared with older 911s where the MC was down under the car behind a steel gravel guard. You can just lean in with the hood open and work comfortably.

Hardest part may be pushing the grommet/seals which hold the reservoir into place. Unlike the older ones under the car, I seem to recall you can mount the reservoir on your work bench before installing.

The symptom you describe is usually thought to indicate that the seal between the front MC pressure chamber and the rear one is a bit leaky, so as you push, some fluid in the front chamber passes by into the rear chamber. A rougher area (due to non-use, so not burnished by ordinary travel) deep in is usually deemed to be the culprit.

I don't know just how a 19mm MC is an upgrade. I know the 914s came with 17s, and 19s were an upgrade of sorts. Haven't heard that messing with MC diameter (don't recall if it is 19 already, or a bit larger because of the power assist) was beneficial on the 3.2 Carreras. With power assist, if you went to a smaller MC you'd hate it, as the brakes would come on too strong with the lightest touch.

[MBAtarga]

Wayne's book shows 4 hrs for the MC replacement. I did this task just the other day - took about 4 hrs total. Reaching back to loosen the hard brake lines was a pain. There's not much room back there to swing a wrench.

[uwanna]

Originally Posted by rekstein

yes you need a new master cylinder same problem upgrade to 19mm master problem solved

an '88 Carrera has a 20.5mm MC

[chrisbruck]

I'm on my 3rd "new" MC trying to get a good pedal. This one is by far the best so I did have a bad string of "new" MC's.

4 hrs is legit to replace it. It's easy to see but a bear to really undo and replace. Very limited swing room to loosen / tighten the line connections and you have to get way up under the dash too to disconnect the brake pedal linkage. I hope I never have to hunch over in the frunk again to mess with mine.

[AVDCAV]

Your all stating 4 hours completely done correct ?
This is a 40 min job including spay bombing the booster,
Maybe 90 mins with bleeding.

Garbage china wrench bend it in a vice zip a small piece out for the brake line,
No bending over. Just get I the bonnet on a pad or blanket,

[chrisbruck]

4 hrs for us older less flexible types....

[osvolant]

Thanks for the many helpful replies.

AVDCAV, what does "spay bombing the booster" mean?

Also, did you guys replace any of the soft lines from the reservoir or just reuse the existing ones?

[chrisbruck]

Reused mine.... they aren't under pressure, just gravity fed.

[AVDCAV]

Spray painting " is bombing. Classic graffiti slang
The things out might as well paint it ,,

[osvolant]

Progress report: I replaced my m/c myself with a new ATE part. Took me at least 4 hours but I'm sure I could do it again in half that. Removing/replacing the ductwork, floorboard, linkage, etc. takes most of the time. The directions provided by Pelican are spot on! The pedal no longer sinks to the floor. Now if I could just get all the air out of the system...

Thanks to all who contributed their expertise.

[steely]

Originally Posted by AVDCAV

Your all stating 4 hours completely done correct ?
This is a 40 min job including spay bombing the booster,

Forty minutes? It takes me that long to find my tools, assuming I remember what I was looking for when I get there.