All brakes stuck on

[Dan]

After 10-15 mins of around town driving it seems like the brake system is building pressure and not releasing the brakes. It eventually gets to the point where the engine wont move the truck. The brake pedal becomes increasingly harder, I can feel the system is pressurized. If I let it sit for a few hours the pedal returns to normal feel and the brakes are relaxed.

Im thinking master cylinder. Any other ideas? What is that black part behind the master cylinder that has some hoses running from it to the power steering pump etc??

Its not the e-brake causing the issue, its all 4 wheels.

Any thoughts??

- Dan