I am one of the unfortunate '89 GTA owner's with PBR rear disc brakes that acts like the 82-88 version with little rear brakes unless you pump it twice quickly.

I understand GM changed the combination valve in mid '89. I have heard the thread size was changed to 1.5 mm, Does anyone know if they changed the valve inside?

Which also brings up the question, do all the '89 owners who are having trouble with their PBR setup have a car made before the mid year combo valve change ? Did GM know about this and change it ?If so why change the thread size ?

 

Just heave the factory POS valve out the window and be done with it. Put in an adjsutable unit like a WilWood.

 

Chickenman,

Glad to hear from ya', your posts are usually quite helpful. IF I wanted to use an adjustable proportioning valve , would you gut the rear side of the stock combo valve to be sure full pressure for the rears was reaching the adjustable valve ?

Bill E.

 

No...don't ever gut the inside of the stock proportioning valve.

With a Dual-Circuit system the front and rear hydraulics circuits have to isolated from each other. If you gut the inside of the stock prop valve the circuits are now in direct hydraulic communication with each other and you can actually pump fluid back and forth between the circuits. Needless to say...this is NOT good!!.

You also acnnot use the stock Prop valve in co-junction with an adjustable unit. It has to be one or the other....not both. Just Plumb in a Brass Tee for the front brakes and plumb the rears directly to the adjsutable prop valve.

TGO has a Tech article on how to install a WilWood adjustable Prop valve.:

https://www.thirdgen.org/newdesign/tech/propvalve.shtml