'77 3/4 ton rear brakes hanging up

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A friend stopped by at work today saying he was out looking for a new proportioning valve because his rear brakes keep hanging up. RR first then the left side. I told him it was probably a wheel cylinder problem but he said they are good. Any ideas here because Im told they dont make proportioning valves for these trucks anymore & I told him Id ask the pros here for help.



I tried the search but couldnt come up with anything...Thanks in advance...