Brakes locking

[BeatDaSHO]

I have a question about my brakes. I have owned this 89 SHO for 3 years and have never locked up the brakes. All of a sudden, within the past week, they have locked up twice on me. I have almost gotten into two accidents because the car just locks up the brakes. I'm not even going that fast at the time. I'm barely on the brakes and they lock up on me. Just for tests, I have gone out and gotten on the brakes to see if they'll lock up and it locks up reallll easy. Anyone have any ideas? Thanks.

Greg

 

[BeatDaSHO]

By the way, I noticed that these brakes started locking up after I did a hard stop from 100-0. I was testing out something else at that point but then I needed to brake hard. (no one was around by the way). Could I have glazed the rotors or something and now they just want to lock up on me?

Greg

 

[pjtoledo]

There are two basic reasons for the brakes locking up. Not enought traction, uneven braking at different wheels. As tires get older, they get harder, and in the wet have lots less traction. Tires that are only half worn may be old and hard enough to be near dangerous in the rain and snow. If a wheel or two has brakes that are falling short of maximum effort, the others can lock up under their increased braking effort.

Perry

 

[BeatDaSHO]

Tonight I went back out and got a second opinion from a friend that knows a lot about cars. He tested it out and he definitely agreed that something is wrong with the brakes and they are locking up wayyy too early. We thought possibly master cylinder although the brake light isn't coming on. We also thought possibly one of the calipers was hanging up. We aren't too sure. Him and I both don't have the tools up here because we are at college so I'm taking it to this real good mechanic tomorrow. We'll see what happens but in the mean time, can anyone think of what this might be? Thanks.

Greg

 

[NotSoSlowSHO]

It it ALL 4 tires that lock up?

 

[BeatDaSHO]

Well tonight when i tested it, it was either the right front or left front that would lock up. It kept changing. The backs didn't lock up though.

Greg

 

[rangerj]

Make sure that the fluid return holes in the master cylinder are not plugged up. If they are plugged they do not allow the brake fluid to return to the master cylinder, that is pressure is not released. When you press on the brake pedal, and then release the pressure, you should see fluid swirl up into the master cylinder from two places. You can clear the holes with a fine piece of wire, but do not get anything stuck in the holes (or anywhere else).

There is also a possibility that the brake lines are swollen inside and not releasing the brake fluid pressure. Pistoins sticking in the bore of the calipers will also cause lock-up, but this would be unusual for all of them to lock up at once. rangerj

 

[BeatDaSHO]

It appears as though the proportional valve is stuck and therefore not letting any fluid get to the rears..........dealer only item? 100 dollars?? sound about right?

Greg

 

[BeatDaSHO]

To those that care............I fixed the problem and it was the proportional valve. It was completely rusted shut not letting barely any fluid to the rears. This caused the car to do a complete nose dive under heavy braking which would then cause a front one to lock up. I put the new proportional valve on and adjusted it correctly. Now the car sits completely flat when under heavy braking and doens't lock up.

Greg

 

[SHOtimer]

I was going to suggest bias plugs, but since you have an '89 that prolly wouldn't be a good idea. No ABS that is. Good job....makes me glad I don't deal with rust...

Doug