'90 SHO Front Brake Calipers Won't Release

[Paul Vincent]

My '90 SHO new front calipers won't releasetheir new pads on the new rotors (unless I open the bleeder screws and release brake fluid). It is a stock system with 162,000 miles, and it was also seizing on the old brakes too (but not as much as the new). I checked that the hoses aren't clogged (fluid freely runs out open bleeder screws with master cylinder cap removed), and I don't believe that I've over filled the master cylinder with brake fluid. After I first made sure the hoses weren't clogged, I took it for a test ride. I went ten or more miles braking every now and then before the calipers locked on to the rotors once again. Could my problem be the porportioning (sp) valve? If not, what??? Thanks.

 

[DHMag]

Paul Vincent:
My '90 SHO new front calipers won't releasetheir new pads on the new rotors (unless I open the bleeder screws and release brake fluid). It is a stock system with 162,000 miles, and it was also seizing on the old brakes too (but not as much as the new). I checked that the hoses aren't clogged (fluid freely runs out open bleeder screws with master cylinder cap removed), and I don't believe that I've over filled the master cylinder with brake fluid. After I first made sure the hoses weren't clogged, I took it for a test ride. I went ten or more miles braking every now and then before the calipers locked on to the rotors once again. Could my problem be the porportioning (sp) valve? If not, what??? Thanks.

when you installed the new calipers, was the piston already back in the housing, or did you use a C-clamp to force it back ?

 

[Paul Vincent]

Originally, I forced the piston back thinking the calipers might still be okay, but a day later (after the trouble increased) I just replaced the calipers with rebuilds, and the result was the same. I'm still screwing around with it, and I released some pressure by reducing the amount of brake fluid. But now I know for sure that I haven't got too much brake fluid in the master cylinder, and yet the front calipers remain clamped too tightly (brakes are far too hot after a moderate five mile drive in 38 degree weather) and rolling resistance is noticable when testing by coasting on a downgrade. I know that calipers can stick, but not to the extent these are under the conditions the SHO is being driven.

 

[clintonk]

Paul, since both old and new calipers are sticking, it's likely they are not the problem. I had the same thing happen on an old ******. The master cylinder was seized. After I replaced that, I had no more brake problems.

Clinton Knight
'90 White 187K
'95 Green 58K

 

[rangerj]

Paul,

Clintonk hit it right on the button. The master cylinder is the cause of your grief.

There are three ports in the master cylinder that can get plugged and cause the hydrolic pressure to not be released.

At each brake line coming from the master cylinder there is a valve that maintains a small amount of static pressure on the breaks.

If either one of these valves gets blocked the amount of pressure on the brakes can be substantial. The amount of static pressure is about 6 pounds.

If one of these were blocked you would have one front brake locked up, and the opposite rear brake locked up.

If both were blocked, all brakes would be locked up. These situation are very, very rare.

Inside the master cylinder there is a third "valve" that is a hole of a specific size that also regulates pressure. This often gets filled with corrosion or "crud", and also will cause the problem you describe.

Try cleaning the master cylinder by siphoning out the old fluid and putting in new fluid. Look at a diagram of the master cylinder for the position of this hole. You may be able to clean it with a fine wire.

Given the age of the car you may be best off replacing the master cylinder. Like many things these days a rebuilt master cylinder is cheaper than a rebuild kit, if you can find a rebuild kit!

Your problem is one of the reasons I advocate flushing brake fluid annually. Try Castrol GT LMA (Low Moisture Activity), or ATE blue or gold. They are both dot 4 fluid with high heat and low moistire activity.

Have a Ford dealer flush the ABS and bleed it. You cannot access the ABS computer in order to set the valves in possion for bleeding.

Hope this helps, rangerj

 

[Paul Vincent]

clintonk and rangerj, Thanks. I really appreciate it!