1983 Mazda Mazda5 - Rear brakes lock

Why do my rear tires lock up? I haven’t driven the carin the last 9 months. It’s a 1983 Mazda GLC front wheel drive with a 5 speed manual transmission. I have already replaced all 4 shocks. And have brand new brakes all around.
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Either your mechanic screwed up your brake job or the combination valve in your brakes that regulates the rear brake pressure is frozen up after 36 years. In any event, it needs to be found and fixed. I suggest starting with the shop that did your new brakes.