On a test drive yesterday, my brakes locked up. I posted here because the Master Cylinder (MC) isn't stock for the M715 it's from a J10 or J20 Pickup.

The MC is new (not rebuilt). The wheel cylinders are new (not rebuilt). The lines are mostly original including the rubber flex lines. The brake fluid was new in May 2019.

The truck has been sitting since July 2019 (I had engine issues).

The brakes locked after driving 10 miles, and when I stopped and applied parking brake the brakes locked up. I had to bleed them a bit to release the pressure.

Some suggested that the free play between the MC plunger and the brake pedal lever needs adjustment (blocks the return fluid inlet when the fluid gets hot and expands). Others have suggested that the return line inlet in the MC might have crud from the old brake lines. One person said that the wheel cylinders could have condensation in them and the return spring couldn't push the cylinder back to its original position.

I didn't check if the brakes were locked on just one wheel or on all wheels (It felt like all wheels were locked though). I didn't have a jack or jack stands with me so I could check, and I was stopped on the road! Not a good place for maintenance*.

Anybody have other ideas or can shed a little light on this for me?

I have been asked to drive in the Veteran's Day Parade, but I can't (won't) do it until I fix these brakes. Stopping is Boucoup important!