Master cylinder problem

racetek · racetek's Garage

I have an issue with my new brakes which I have never come across before. I think it is an issue with my new master cylinder but I thought I would post here to see if I have missed something. (I hate changing master cylinders).

I have a completely new brake system. New M/C, rebuilt calipers and disks, new bake lines and fittings. I have bled everything as per many of the posts here, made sure the pads are sitting on the disks and bled them again.

When I jump on the pedal after the car has been sitting for a while, the pedal is very firm, as I would expect it to be. If I gently rest my foot on the pedal and press with just enough effort to move the pedal it will slowly sink to about 50% of its travel and then become firm. If I let the pedal back up and stamp on it firmly, not with great effort, just firmly, it is firm immediately and feels great. Another soft press on the pedal and it sinks down again. It feels as if the fluid in slipping by the M.C seal but I won't know unless I replace it.

Has anyone experienced anything like this.

Joe Bob · Joe Bob's Garage

Are you loosing fluid? Boosted system/SC?

racetek · racetek's Garage

Hi Joe Bob,

No fluid loss, whole system is dry, no leaks anywhere. The car is a 76 911, no servo, A type caliper up front and M at the rear. All the bleed nipples are on top. I just tried them about 5 minutes ago, after sitting for 24 hours. A light jab on the pedal and it is as firm as it should be, but press it very gently and it sinks to 50% before becoming firm.

racetek · racetek's Garage

Just had another little play and with the help of my good wife (as opposed to my bad one), I have discovered something which may help the diagnosis. I got my wife to jab the pedal and I checked all the brake discs and they were all locked up. I then got her to lightly press the pedal until it fell to about 50% and I rechecked the disks. The front were locked up but the rear were free,so I was loosing the rear brakes. Looking more and more like the M.C.

DRACO A5OG · DRACO A5OG's Garage

When you bled, did you go all the way to the floor? This is a No No, you may have damaged the MC. I would place a block behind the pedal to insure this will never happen again. If that is the case of course.

MC should be under warranty BTW.

Targalid · 12-11-2012, 11:44 AM

I had the same problem when I replaced pads and restored rear calipers. I think your problem is the brake line on the top of the master cylinder (this is if you have car of the same vintage as mine with the MC next to the steering rack under the car). After you remove the skid plate, several brake lines need to be bled. The one that is really hard to reach, the top one, seems to be the culprit. Crack that one and bleed the brakes again. I was amazed how much air came out of that fitting. It carries fluid to the rear of the car if I remember correctly. Because the line goes up and then down again in a loop, air is trapped at that spot. Keep at it. Eventually you will get the air out of the lines.

racetek · racetek's Garage

some more info before I spit the dummy and yank the M/C out again. When I jab on the pedal everything is OK, when I press the pedal slowly it fall 50% to the floor AND THE FLUID LEVEL IN THE RESERVOIR RISES. When I let the pedal up to its resting position THE FLUID IN THE RESERVOIR DROPS BACK TO NORMAL.

m/c?

DRACO A5OG · DRACO A5OG's Garage

Quote:

Originally Posted by Targalid

I had the same problem when I replaced pads and restored rear calipers. I think your problem is the brake line on the top of the master cylinder (this is if you have car of the same vintage as mine with the MC next to the steering rack under the car). After you remove the skid plate, several brake lines need to be bled. The one that is really hard to reach, the top one, seems to be the culprit. Crack that one and bleed the brakes again. I was amazed how much air came out of that fitting. It carries fluid to the rear of the car if I remember correctly. Because the line goes up and then down again in a loop, air is trapped at that spot. Keep at it. Eventually you will get the air out of the lines.

He is talking about this one behind the stone guard:

racetek · racetek's Garage

Found the problem. Took the M/C out and bench tested it. The rear brake section was pumping fluid back to the reservoir. This was caused by the rear rubber cup leaking fluid past it and back to the reservoir. when you hit the pedal with a bit of force it would force the cup to expand and seal. I changed the M/C to a new one and the system now works fine. Thanks to everyone for their help.

David

Joe Bob · Joe Bob's Garage

Could that be fixed with a rebuild kit or was the bore scored? Also......proper floor board and stop is essential.

racetek · racetek's Garage

I had a spare M/C but the original was supposed to have been rebuilt. The bore is perfect and the seals look good, but it just didn't work.

Anyway, it is now in the spares box and I will fit a kit to it when I find one for a reasonable price.

dave 911 · dave 911's Garage

Quote:

Originally Posted by racetek

. . . Anyway, it is now in the spares box and I will fit a kit to it when I find one for a reasonable price.

Where does one find a rebuild kit for MC's? I'm not finding them available anywhere anymore, for either the 911 or a VW beetle. I spoke to a rep at Wolfsburg West (the 'Pelican Parts' for VW's), and they said they don't carry them anymore for liability reasons, the same as what I've read in other posts here.

zippy_gg · zippy_gg's Garage

Back in the 80s I tried to rebuilt the MC on my 924 with the FAG kit and it didn't hold.That was before there were forums otherwise I would have learned beforehand to just bite the bullet an get a new MC as the stock ones cannot be rebuilt for some reason.