I do not think you have a problem with the valve. Here is what you need to check:
From donig this for 38 years I find most of the time the problem is with the rear shoes,
I find very small hair line cracks in the shoes. This is the most likely. There can be no cranks what so ever, the drums need to be turned and have no marks on them except for the groves the brake lathe leaves. The wheel cylinders must have free movement and no leaks. The front brakes also need to be checked. If they are not working properly the rear will lock up. Proportion valves are very uncommon but can not be ruled out. But first all other items need to be checked. Please let me know if you need me to help or clarify my information. Best to you .. Jim
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I need to know for sure do both of the rear brakes lock up or is it just one of them?
BOTH WHEELS LOCK UP(REAR)