dmruschell

I was driving my 56 Olds 88 this morning, and at one point during the drive, the feel of the brake pedal changed. It still stops just fine when the pedal is pressed, but it has a lot more free play at the top of the pedal travel, and the pedal extends closer to the floor before the hard braking occurs.

What would cause this kind of sudden change?

Olds Scott

Does your car have power brakes or manual? Brake shoes might need adjusting.

dmruschell

The brakes stopped working completely, so something is messed up. Time to call the mechanic.

Tedd Thompson

Is your brake fluid level high on the stick If power brakes your booster booster line or valve may have failed or developed a leak. Low or no vacuum will cause the problem you have described.

With the engine off, brake peddle depressed, then start the engine. Does the peddle depress more? If not you are not getting any help from the booster or could be a seal is bad in the Treadle- Vac . ...... Tedd

m371961

If it happened all at once I would look for a leak.

dmruschell

It has manual brakes, so no dip stick. There is a square-ish plug on the top of the master cylinder that I couldn't open with my fingers. The free play kept getting larger in significant intervals until the whole pedal is free play.

Either something is stuck in the master cylinder, the seal on the master cylinder is not sealing, or a wheel cylinder is leaking badly and sucked air into the line.

Olds Scott

Sounds like the whole system should be checked out. From wheel cylinders, hoses, hard lines to master cylinder.

dmruschell

After getting a tow, the brakes work again.

2 leaky wheel cylinders were replaced last year. All hoses were replaced last year. I did not see any signs of a leak around the wheel cylinders.

The master cylinder did appear to be leaking from under the rubber boot cover. After looking up prices of brake master cylinders, I'm glad my car doesn't have power brakes.

I've read that the pedal needs some free play because of a valve that adjusts for fluid expansion. How much free play should the pedal have?

Ozzie

The shop manual says minimum 3/16" measured at the pad. Be careful; the piston of the master cylinder may be binding in its bore. Whatever the problem is, it should be resolved before the car is driven.

dmruschell

Thanks. I just ordered the shop manual since I'm starting to work on this car myself.

I had considered that the piston might be binding in the bore and causing the brakes to not engage until further in the pedal travel. But, then I had thought that that would mean the brakes were a bit engaged all the time, which didn't seem like was happening. But, if the brakes failed because of brake fade (I've never had that happen on this car before), then maybe the brakes were engaged slightly more than they should have been.

Either way, between the possibility of a stuck piston, a visible leak, and no record of the last time it was serviced (it wouldn't surprise me if its 60 years old), I'm planning on replacing (not rebuilding) the master cylinder.

When I got the car, the brakes worked to keep it from rolling down a hill. However, when my mechanic was rebuilding the engine (and doing work to get it drivable), he said there was basically no brake fluid in the master cylinder. If moisture got in there instead of brake fluid, then it needs replacing.

dmruschell

Well, the old master cylinder was definitely causing the problem. When I checked the brakes today, they felt like they always did. I pressed on the pedal, and the next time I pressed on the pedal, it had more free play than it did when I first checked it. This was also demonstrated once I got the master cylinder out of the car.

So, I have the new master cylinder installed in the car. My shop manual is being shipped across the country and won't get here until next week. The replacement master cylinder from Fusick doesn't feel like it has any free play at the end of the piston travel like the original does, though I haven't filled it with fluid and bled the brakes yet. I figured I'd be able to feel when the piston is actually engaged. I just figured I'd check and see if there is a proper procedure for adjusting the piston travel of the master cylinder in relation to the pedal travel.

dmruschell

I figured I'd post the conclusion to this saga for anyone else who comes across it in the future.

I received my shop manual today and looked at the adjustment for the master cylinder. It needs to be adjusted so that there is some movement of the pedal before the brakes engage (as previously mentioned the 3/16 inch specification is mentioned in the manual). This movement actually pumps fluid out of the piston bore back into the reservoir through a hole and allows for fluid expansion due to heat as well as replenishment of fluid if some leaks out of a wheel cylinder, etc.

The rough adjustment I made while installing it last week was pulling the piston back a little too far. This could be seen when releasing the brake pedal. It would move back towards its resting position, then catch a bit, and then slowly move back the last little bit to full rest.

I made the adjustment under the car, since I could work the brake pedal linkage by hand, observe the return of the brake linkage, and I could actually hear the fluid being pumped back into the reservoir before the brakes engaged. I adjusted the brakes so that there was about as much travel as possible without the brake pedal "catching" when returning to rest. I took it out for a test drive, and all seems to be well.

The (I'm guessing) original master cylinder piston definitely wasn't returning all the way, so the first couple of inches of pedal travel wasn't doing anything. This also meant that the brakes were dragging excessively, and there was no accounting for hot fluid expansion, since it wasn't returning enough to uncover the hole leading to the reservoir.

Thanks again to everyone who chimed in, and hopefully this helps someone in the future.