1983 F250 with 351 and 3spd automatic.

1) When brakes are not pressed, there is a hissing sound coming from the engine compartment. Press the brakes and the hiss goes away.

2) Press the brakes and dash lights inside the cab come on. Un-press and they go off.

Thoughts?

80s

 

Hissing is air in the system or the fluid is old and broke down. I would chase wires to see if when you press the brake pedal you're not grounding on any wires.

 

i agree on both points above. If you mean the dome light comes on when you hit the brake then you have a short somewhere.