Brake drag. Need help.

Actually caps are Not vented "exactly". The gasket serves as a bladder to "follow" the loss of fluid as the shoes / rotors wear. The metal cap is vented into the top of the gasket, but the gasket/ bladder completely seals the master. I've not experienced this, but I can see where it would be possible for heat in the engine bay to expand things enough that pressure would build in the master. Easy.......use a syringe/ etc to remove fluid. Drop it down 1/2" or so and try again. Normally, even if the master is "brimmed" the bulge in the tops of the cap should be enough "airspace"

I would be VERY suspicious of a long pushrod, or something "hanging up" so that master is not fully returning.

If return can not be fully confirmed, "this can get messy." Put a pan and LOTS of rags around / under the master, and leave the cap off or maybe lay it on "loose." Use a helper if you can. When you apply, then release brakes, you should see a "spurt" of fluid coming up out of the ports in the bottom of the master at each end. These are sometimes violent enough to spill over. "Go easy" with pedal release until you see what they will do

As said above there should be NO residual pressure in system with pedal relaxed.

You can easily check of the rotors don't have clearance. Remove one caliper at a time, and devise scrap wood whatever you need to retract piston. "Go easy." Make sure that when reinslalled there is plenty of pad clearance.

The pistons/ calipers do not have ANY method of retracting pads operationally, except for the tiny movement in the piston seals. They ?? deform to one side as pressure is applied, and are supposed to then return causing the pistons to "walk" in a tiny amount. Things like poor seal quality, poor condition of pistons/ calipers etc and various misalignment problems can diminish that little return