If it is all 4 breaks that don't release then it doesn't sound like the parking brake is the problem. Check if you have an aftermarket brake master cylinder with an adaptor between the booster and the master cylinder, it is well known that some of those old adaptors are a bit tight/get a bit tight when the car warms up and stop the master cylinder releasing properly and then the brakes are always slightly on. I had exactly this problem with my Pantera since I bought it. Changing to the right master cylinder without the adaptor fixed it. Some people also talk about collapsed brake lines not releasing pressure, but I guess this would not affect all 4 corners at once. I haven't experience that myself..
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