1973 Dodge Truck brakes

The truck is not used often. Now, the brakes stay engaged after pushing pedal and stay engaged until I pull pedal back manually. The rear fluid chamber was empty so I replaced the master cylinder and bled, still the same problem. Is there a spring or something worn out in the pedal mechanism? The system is power with disc in front. Brakes are in good condition, stop fine, just need to pull the pedal back up to release. Thanks