Help: 96 Grand Am - excessive brake pedal travel and grabbing

Sounds like a leaking line somewhere. The grab may be the back brakes engaging after the fronts do not.

Any history on the development of this problem Bill? Did it simply start one day? Was there any work performed on the car prior to the onset?

Ok I'll check for any leaking tomorrow. However, The reservoir never goes down in fluid level.

It also does this: with the motor off, the pedal feels tight at the top. It is only when the motor is running that the 1-2" pedal sink take place.

Could it be a faulty air valve >>

The master cylinder could be leaking internally. Another though is the brake hoses could be weak, allowing them to belloon out. Last idea is a stuck caliper.

Well - the easy answer is do what you did last time. Blah! That sucked, didn't it? The slop at the top of the pedal implies a leak and air getting into the lines. The first thing I'd do is to inspect the rear wheel cylinders, all of the brake lines, etc. for any sign of fluid leaking. I'd be looking for even small signs. I'm sorry - I don't know if the '96 Grand Am has drums in back or disks. If it has disks, then you can obviously ignore the wheel cylinder idea. Still - look for leaks.

If you cannot find any leaks, then I think I'd go for the master cylinder next. In fact, I think the master cylinder might be closer to the front of my mind. Make sure you bench bleed the new cylinder before you install it in the car.

The brake grab is what is throwing me. I've only encountered severe grab when a bolt backed out of a caliper and allowed the caliper to rotate into the wheel upon braking. Now, that was grab! Absent any other ideas or thoughts with respect to the grabbing, I think I'd hit the master cylinder. $40 and a core.

Another thought Bill...

How old is the brake fluid in the car? Brake fluid assumes moisture from the atmosphere and you can get into a situation where with water in the lines you have no pedal, since water does not compress. At a point however, you will begin to compress the brake fluid that is in the lines. Perhaps that could explain the grabbing and the soft pedal.

If your fluid is more than a couple or a few years old, or, if it looks dark, then purge it and replace with new. You can do this by simply gravity bleeding the brakes and keeping the reservoir topped off, or you can pump it out with the pedal while keeping the reservoir topped.

Yes and no.....water does not compress, therefore he WILL still have pedal.

At a point however,

No.... brake fluid will not compress. Brake fluid, like water, is a liquid that does not easily compress. Water only compresses when it becomes hot enough to turn into a vapor/gas.

Perhaps

My guesses, in no particular order, would be vacuum booster, front brake hose or binding caliper. It would be interesting to know which wheels are locking up.

Dave

Damn Dave - ya spanked me hard! Well, at least this one will be an interesting one to see play out.

Probably a hole or other leak in the boosters atmospheric valve.

Heck, if I'd meant it to be a spankin', I'd have used some adjectives.;)

Dave

Thanks for the replies, guys.

As for the grab, I will go for one more short drive to see what wheels are grabbing first. Then I will put the car up and inspect everything.

One last question: if I do replace the master cylinder, it there any concers with removing the ABS unit that is directly attached to it?

Uh, Dave - there is a slight error in your otherwise good advice. When water is heated enough to turn into a vapor, it doesn't compress, it expands. Try boiling a sealed container of water and you will see that! ;-)

The valve is not replaceable. I will assume no one adjusted either of the rods in order to make it do what it is doing. Look for a diagram on the net of how a booster works. When you push on the brake pedal, it opens a valve that lets atmo. pressure push on the diaphram which helps push on the rod to the MC. If too much air gets in too fast it will do exactly as you describe.

True, but, it's expansion is not enough to overcome the high pressures in a brake system. Vehicles in the mountains often loose their brakes because the moisture in their system vaporizes and the pedal goes to the floor. In everyday driving, there usually isn't enough heat generated to be a problem.

Dave

Not really - just don't horse anything. But... better advice has been put forward by Dave and some others - namely to investigate the vacuum booster first. Take a look back at the couple of responses that directed you that way and look there first.