Brakes seizing up on 97 Geo Prism

Several months ago our 97 Geo Prism started acting strangely... it seems that the brakes are applying themselves somewhat randomly, which manifests itself in symptoms like the car starting to shake (typically at high speeds), the brake pedal becoming very stiff, etc. The problem typically comes on gradually and gets worse, sometimes making the car practically undrivable, other times letting you get to your destination.... and the next morning everything's fine. You can drive the car for days at a time with no sign of trouble, and then have it happen again. We've verified that when the trouble occurs, one to four of the wheels are actually locking up so they can't rotate easily.
We've had the car in to our local mechanic numerous times. We had the master cylinder replaced; then the brake hose assembly replaced; then a vacuum ck valve to booster installed, proportioning valve to brake line installed; system bled & fluid added; power brake booster installed...
Our mechanic was at a loss because the problem kept occuring. We took it to another shop and they said that when they 'bled' the master cylinder, the pressure went away and the problem was alleviated. Based on their suggestion that the problem was the master cynlinder, we took it back to our original mechanic and had him replace the (new) master cylinder. Alas, the problem still exists.

I'd really like to get some more miles out of this vehicle-- I think the engine's got it in it, but we're at the point that if we can't get the brake problem worked out, we might just need to get another car, as we can't feel confident about driving it more than 20-30 miles away. :(

Does anyone know what the real culprit is or have an idea of what we can do from here??! THANKS. I have to guess that all the checks have ruled out frozen calipers/wheel cylinders if rear drum brakes and all flex hoses.

I think that once the problem was fixed and you replaced the master again that the master cylinder maker has a bad/defective product. It's happened to me and took a lot of inquiring as to what was the real problem as my work was fine but the shaft in the master cylinder was not machined properly for the push rod that applies pressure to it and when the car warmed up the brakes started to drag up barely being able to move it?? Confused as the customer calls me, I go to the car and it's fine?? (cooled off) A different new master cylinder solved the problem forever after some hair pulling to figure it out.

Forget the brand of the part but it was new - not rebuilt and made in Italy. That parts company knew of the problem and it was just one factory as it turned out,

T
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Tom Greenleaf - MetroWest, Boston
http://acsource.net/acforum/index.php









Just remembered. The company was "AutoSpecialty"

T
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Tom Greenleaf - MetroWest, Boston
http://acsource.net/acforum/index.php