1990 C1500 Brake nightmares (need someones experiences here...)
 

Has anyone run into "intermittent" brake system problems on 1990 (or thereabouts) C1500 SWB 350 5.7?
Very hard to push pedal half the time, then brakes seem kinda spongy and a bit grabby other times. I have systematically replaced most parts of the system and bled the lines countless times. 2 parts that I haven't changed are the Combination valve and Iso/Dump
Valve (for the ABS?). Can't be sure if they have problems or not.
Anyone have similar issues around this year model?
Anyone know how to test the combination valve and/or Isolation/Dump valve?
Manual says to hold open the Comb valve while bleeding, does anyone know why?
Man, I'm lost at this point and any help would really be appreciated?

PS: I'm new here and already love the site! Great job all!!!

Got 179000 on her and she still runs great (just doesn't stop too well at the moment...)

I have the same problem and I suspect it is the dump valve. I am still researching this.

Old post?

My 90 model had the spongy break syndrome and the grabby breaks at times. I replaced the Dump vavle and blead the breaks and never had a problem. Bought it in 97 with 98,000 and sold it in 04 with 207,000. Before I changed the Dump valve I replaced the master cylinder, rear break cylinders and front/ rear brake pads. Nothing worked until I replaced the Dump valve. just a side note the Dump valves were notorious for acting up. Hope this helps...

Where do I get one?
 

thanks for your answer. I feel like taking it out, because I don't know the condition of the rest of the system.

Dealer wants $350+ for one. Where did you get it? Hate to go to a recycler.....never know what you are getting.

I am either going to replace or bypass it. I am having trouble finding fittings to by pass it.

I got mine at the dealership. Call aound to different dealerships both chevy and gmc. Look on Ebay. You could possibly try a salvage yard. Do not bypass it. That will cause computer issues as I recall someone saying.

Well I have not idea if the computer is working either. have had a red light for YEARS. And I feel it is a pretty crummy system. Will keep looking. thanks