Proportioning valves

[chadcampbell00]

I bought a '70 Jimmy a few weeks ago and the rear breaks have a tendency to lock up at more sudden than normal breaking. It's pretty bad on gravel and causes the rear end to slide sideways. The previous owner installed all new braking components (booster, master cylinder, drums, shoes and the rubber lines). It has drum brakes front and rear. I noticed that it has a proportioning valve but I thought only vehicles with front disk brakes had that. I'm not real familiar with proportioning valves so if anyone is I could use some advise. Did these models come factory with them or was it added? Should I replace it or bypass it? I have seen some adjustable ones online, should I try one of those? Or maybe my problem has nothing to do with that. Both rear wheels lock up so I don't think it's a cylinder problem and they should be self adjusting. They release when I let off the brake so I don't think they are too tight anyways. Any advise?
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