[funbnme]

I believe the piston in the master cylinder is held in place with a snap ring. Pull the master cylinder and make sure the snap ring that holds the piston isn't twisted.

If it's twisted (I had it happen to me once), a little bit of pressure would be applied to brakes and the pads would heat up from dragging and that expandes the fluid in the caliper. The fluid expansion winds up applying the brakes.

Also use a caliper to measure the length of the pushrod and make sure it's not a hair too long for the master cylinder.