Stoping power decreased

Ok, all of a sudden my stoping power has really decreased and I really have to apply pressure to stop the car. When it stops the rotors shudder. I can still drive it, it's not TOO bad but I don't have good stoping power. I have to push the pedal all the way hard to stop the car. I'm sure my rotor's are not warped, but they are old. I checked the brake fluid, that's OK. The brake booster and master cylinder were replaced last year. What could be the problem?

Originally posted by TXBill
Is your car turning off with the key? If it is that rules out the vacuum pump failure and possibly the vacuum leak.

Yes the car turns off with the key normally. I guess it's the master cylinder then? Thanks Bill.

[be459]

Along the vacuum line, there is a valve not too far from the booster. Can it be the cuplit? I do not think you can buy just the valve. You have to buy the entire vacuum line. Check it with a vacuum gauge. Make sure you do not have clogged flex hoses.

David

[MarkM]

I recently replaced the tortion bar bushings, and had to detatch the master cylinder and brake booster and move it forward (while still attached to brake lines) in order to access the bushing. When I put it back together, my brakes were terrible. When shutting off the car, I heard a vacuum leak from junction between master cylinder and booster. I loosened nuts, cleaned, put silicone grease on "O"ring, and reassembled. Then, the car would shut off ok, but I would still have brake problems...intermittently no power assist.

I noticed that the "O" ring was a standard round "O" ring, and did not appear thick enough to provide good seal. I ordered a new "O"ring, and replaced the old one. The new one was different....rather than having a round cross section, the new one was square and a little thicker. After installing new seal, I have not had any problem whatsoever.

So, you might be getting leakage through this seal. $2.50 at dealer, easy to replace without detaching brake lines...just pull master cylinder forward enough to get clearance....make sure both mating surfaces are clean. Put a little silicone grease on seal to help it seal.

good luck,

Mark

[JimSmith]

SV,

I have not heard anyone explain why the shuddering and so on you describe will go away if you address the master cylinder.

Is the feeling coming from the front brakes or the rear? Do you have to pump the pedal to get the car to stop? Does the pedal sink slowly to the floor? How do you know the discs are not warped? When was the last time you changed pads?

I would be concerned as the shuddering is originating at the brake on either both front wheels, or at least one of them. A vacuum leak would explain needing to push really hard on the pedal, as you now have to make up for the lost assist from the vacuum booster. But that does not explain the shuddering either.

I once had a fuel leak that soaked the left front brake with Diesel fuel, making the brakes pull severely. It also introduced a shuddering. I have also soaked the brakes with water and made the pads seem useless until they dried out. This also introduced a shuddering that went away once the brakes were back to normal. Anyway, my point is I think the problem lies in the brake assemblies at each wheel, as the vacuum leak and master cylinder only address half the symptoms. Good Luck, Jim

Originally posted by JimSmith


Is the feeling coming from the front brakes or the rear? Do you have to pump the pedal to get the car to stop? Does the pedal sink slowly to the floor? How do you know the discs are not warped? When was the last time you changed pads?

The shudder is coming from the 2 front rotors. I don't have to pump the pedal to stop it. The pedal doesn't sink to the floor. I really don't think the rotors are warped because the shudder doesn't happen everytime I hit the pedal and stop, it happens randomly, and also the pads have been changed when necessary since I got the car. I got the car about a year and a half ago, and the shudder is a recent thing, going on for maybe a month or two. The last time I changed the pads was about 8 months ago. I just have to put more pressure on the pedal.

Thanks for all your help guys.

[Ken300D]

First of all, let me say that I think the vacuum boost system is the first thing to check.

Also, if both rotors on the front are warped, it is possible for them to go "in synchronization" at times and at other times be perfectly out of sync. What I mean by this is the warping of one rotor causes a pulsating feeling. Well, you add a second rotor, that pulsating will either be in phase (larger overall pulse) or out of phase (they cancel). As you go around turns the "phase" changes, resulting in the pulsating being worse at some times than others.

Warping is typically caused by a frozen caliper piston on one side of the caliper assembly. This means you are applying pressure against only one brake pad, generating heat on only one side of the rotor. Uneven heat causes the warping.

This also can reduce stopping power.

There are other reasons for brake rotor warping, but I think I've mentioned the most common. You may be looking at a front end brake job with a rebuild of the calipers. An inspection with the wheels off might show one brake pad considerably more worn than the other on each side. If so, that points to a stuck caliper piston on the side where the pad isn't wearing.

Ken300D

[psfred]

SV

When was the last time you replaced your rotors? MB recommends new rotors every third pad replacement, and my mechanic replaces them when he sees or feels a ridge at the outer edge where the pads don't quite reach.

There is a reason for this -- when the rotors get too thin, the ATE type caliper will bind -- the backing on the pads will contact the anti-rattle sping and cock the pistons. Results in VERY hard pedal (as the pads are only being forced onto the rotor at the bottom edge) and will eventually cause the pistons to seize, dragging the brakes and roasting the rubber dust cover and piston seal. The caliper will then either stick and drag, or stick and not work (hard pedal), pull hard, or leak fluid along with the above.

I suspect you either have stuck calipers, undersized rotors, or contamination of the brake pads with oil or brake fluid -- or the pads are completely gone and you are running on the backing plate. That usually causes borrible grinding, though (I've done it!).

Pull the wheel and check the pads, or have it done pronto -- bad brakes are an accident waiting to happen!

Peter