CrossedUp

Around 2-1/2 years ago I replaced the master cylinder on my '65 because, while under the dash for something else, I noticed that it was weeping a little fluid into the passenger compartment. It is a stock single master for manual brakes. The system bled just fine and has been performing flawlessly on my weekend drives ever since. However, on the most recent drive the brakes began misbehaving. On initial application the pedal goes down an inch or so before it becomes firm and at the same time one of the rears produces a loud humming sound (not a squeak). If you release and immediately reapply the pedal it returns to its normal firmness right near the top of the travel and the humming goes away. When I got the car the PO told me he had the calipers converted to the o-ring style seals if that makes any difference.

Has anyone experienced this before? What would cause a pad to chatter like that? Could it be a piston hanging up in one of the rear calipers?

I think I'll begin with a bleed and check for air in the system before I start taking things apart.

Frankie the Fink

Sounds like a sticky caliper to me - not uncommon at all...
Often on rear brakes a problem won't cause the car to pull one way or another like front brakes would...

GTOguy

I agree. Sticking piston or leaking seals or both. Time to get it in the air for a visual and mechanical inspection.

jim lockwood

A brake pedal which goes low on one application and is normal height on the next application can also be caused by bad wheel bearings.

The bad bearings allow the rotor to wobble, knocking the caliper pistons back in their bores. First application of brakes after this happens results in a low pedal as you move enough fluid to put the piston back in contact with the rotor. If no rotor wobble happens prior to the next application of brakes, the pedal will be normal height.

The humming sound? My only guess is that it could be a rotating piece contacting a non-rotating part because of the worn/loose bearings.

When you figure out what's going on, please post the findings. I'm curious....

CrossedUp

Thanks for the quick responses gentlemen.

It sounds like bleeding isn't going to fix anything. I'll investigate and report back.

Avispa

For sure, bleeding won't fix your problem if the pedal returns to firm after the first pump. If the pedal firms up, there is no air in the system. It was pure coincidence that your problem started after the master cylinder change.

This may not be your problem, but my experience with o ring caliper seals was not a good one. They played tricks on me right from the beginning - pedal went down halfway on fist push after driving a few miles and on the second and subsequent push was firm right to the top. I expected this exact thing when I put the o ring seals on the pistons - the friction was so much that the springs couldn't push the pistons back out. That was 15 years ago; maybe there are better o ring kits these days, but I went back to lip seals and never had that problem again.

MikeM

I would begin with first things first. The master cylinder is/was leaking. Fix it. If you still have problems, go to the next symptom.

Factoid

Ensure all your lug nuts are tight. A wobbling wheel can cause your problem.

Railroadman

The way I read his first sentence, it was leaking, he replaced it.

Gavin65

This, get the car up in the air and check for play in the rear bearings.

MikeM

Yeah, you're right.