I decided it was time to renew the brake system on my 43-year-old car. So, I bought new hoses, brake rotors, rebuilt front & rear calipers and a rebuilt pressure regulator from PMB. I also bought an NOS 17mm master cylinder from a friend.

I just finished installing everything and bled the brakes. I used a pressure bleeder and my son working the brake pedal. This usually does a great job of blowing any air bubbles out of the system. I also set the venting clearance on the rear brakes, as per the factory manual.

Unfortunately, the brake pedal goes right to the floor. I'm not getting any air bubbles out of the calipers and there aren't any leaks anywhere. And, even if there were still air bubbles in the system, the pedal shouldn't go to the floor with very little resistance -- it might feel spongy, but shouldn't go to the floor. I've worked on plenty of brake systems before, but have never had this problem.

Could the master cylinder be defective? Like maybe it was sitting on the shelf too long? There was no rust at all on the MC, and it still looked brand new. The brake pedal was able to push the caliper pistons out to the rotors, but it won't provide enough pressure to hold the rotors -- I can turn them by hand, but do feel drag.

I used brand-new DOT4 fluid, since it is what I had on hand. Could this have caused the problem? Both the factory manual and the owner's manual say to use fluid that meets SAE 1703 (which this fluid does), but neither say which DOT rating. I always thought DOT 3 and 4 were interchangeable, but could this have caused or at least contributed to the problem?

Any chance this will get better once the master cylinder seals have a chance to soak in the brake fluid? I sincerely doubt it, but thought I would ask.

I tried calling PMB, but they're closed today. I'll call them tomorrow, but was wondering what the World had to say about this. I'm guessing this means I need to get (another) new master cylinder, doesn't it?

Id try to foot bleed it also, I always have after the pressure bleeder.
that solved my problem.

We did both, pressure bleeding and foot bleeding. Didn't make any difference.

Did you bench bleed the MC before installing it?

If not, can you bleed the MC AFTER install? Curious.

You can bench bleed after the install went south. No idea why it wouldn't work unless you tore something up inside the MC while dry pumping it. The real question to ask is do you want to risk it? In the big scheme of things a new MC is cheap.

Yes, just slip a clear hose on the outlet side of the MC and pump the fluid back into the reservoir. If it's a new MC you can do full strokes. Eventually the bubbles will quit coming out of the reservoir.

I have to ask. Are all of the bleed valves on your calipers pointing up? There are right and left calipers and if you have them on the wrong side, the bleeder will be on the bottom, so you won't be bleeding any air out. Just brake fluid and frustration.

I learned that the hard way, only took me a week to figure it out.

All of calipers have two bleed nipples, one on the top and one on the bottom. So, yes, all calipers have bleed nipples on top.

That's a good thought, along with the bench bleeding, but I think both are inaccurate. I don't have a spongy pedal, I have no pedal -- pedal goes right to the stop without resistance. A spongy pedal would (should?) build up at least a little pressure before reaching the stop.

In a further sign the MC is probably toast, I found a small amount of brake fluid in the bottom of the pedal cluster. It looks like it leaked out of the rubber boot where the pushrod goes into the MC.

I did not dry pump the MC, but who knows how many times it might have been before I got it, or what other abuse it might have suffered.

I'll call Eric as soon as they open tomorrow. This just sucks because A) it's a PITA to install and B) I have to buy the same thing twice.

Before you totally give up on it, you could try and submerge the whole mess in Tupperware filled with brake fluid. Pump it while submerged. I've done it with success on bug MC's.

The leaked fluid says that the MC is bad. Replace it.

The rear brake venting clearance spec is too large; set the clearance to half of the listed spec for better feel.

--DD

That's the safest option and one I'd be most apt to take. Brakes are pretty important.

FWIW, I've seen old assembly lube dried on the cylinder wall that prevented a good seal. Total submersion and actuation can soften and wash it out so it seals again.

I also understand not to pump the brake pedal without the pedal board in place as you can "over depress" the the brake pedal causing the seal in the MC to be damaged.

I bought a NOS 17mm MC too. Turns out the seals inside had failed. 43 years of sitting on the shelf will do that.....

I could never get pressure to build. Took the MC off and took it apart and found a split seal. Can't find 17mm repair parts either, so I had to buy another MC. I suggest that you remove it and disassemble it. They are really simple devices.

19mm MC is cheaper and has parts available, might be the way to go.

I think you should first check that the push rod is not too long.
If it is too long the piston cannot come back far enough to allow fluid to enter the MC.
A side effect of this is it may allow fluid to leak from the boot end of the MC.....which you have seen.
No fluid in ? impossible to bleed.
Do you have a strong flow of fluid with every stroke when opening the bleeders ?

I'll disassemble this one at some point, it probably suffered the same fate. I had a hard time trying to get the front calipers pumped out to get the pads to meet the rotors, so this MC was probably bad right from the start.

I thought about the 19mm MC, but would rather stick with the 17mm. The brake pedal is hard enough as it is, don't really want to increase the pedal force by another 25%.


I checked that before I even put fluid in the reservoir. It has the required 1mm of clearance.

I have to ask how did you bleed the brakes by working the brake pedal if ypu say it goes all the way to the floor without resistance?
Is the pushrod there, did you check after the install?
Are the feeding lines connected well with the rubber seals? That's the job that usually brakes a finger or two.
I would expect that even a faulty internal seal will show some level of resistance. If you feel no resistance and the pushrod is there, then I would assume the MC is sucking in air instead of brake fluid.

It's a freak of nature.

Get a new MC and be done with it.

Just saying.....

Your guess is as good as anyone's on the integrity of the NOS M/C. NOS from 40 years ago or NOS from 5 years ago?

That said I had exactly the same symptoms on my fresh 19mm M/C swap last fall. That was until I bled the calipers as outlined on the PMB site. Have bleed a lot of systems on 2 and 4 wheelers and I didn't even think it would have worked having always had success with either a pressure bleeder or the 2 person method.. But it did and the pedal came right back. "Just sayin"

Of course this all presupposes that you got the tricky install of the supply lines into the M/C correct.

Id just buy a new master from PMB.
the 19 is fine honestly. it takes nothing to stop my car.

PMB rushed me a new master cylinder. Just finished installing it and bleeding the brake system. Everything's working like it's supposed to. So, the problem was a defective master cylinder.

Kudos to Eric. He's helped me out several times with brake/rotor issues. Even provided tech support on some rear calipers that were rebuilt by someone else. Class act.