Brakes lock up and pedal becomes real hard after driving for a little while

Sounds like the brake shoes are out of adjustment. Back off the adjustment wheel for the shoes so you can just freely spin the tire by hand. Repeat for the other 3 wheels. Hope that helps! Jim

Brake hoses may be collapsing internally , if you can't get fluid from the bleeder when this happens then that may be it .

impurities in the system can cause things to lock up. I would flush the system completely and clean everything including brake wheel cylinders or just replace them .