AC, that type master was commonly sold as a replacement by many Pantera parts vendors in years past. There's often a bolt pattern adapter with a poly bushing inside to sort-of guide the short actuating rod. The constrained bushing squeezes down when things heat up, causing the rod to stick and brakes to lock on. Reaming out the hole fixes things- I've done it to afflicted cars on weekend runs using a Swiss Army knife.
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