1991 Mack Midliner CS300

Inside the cab on the dash I have two indicator lights for the air brake system.
1." BRAKE " indicator light - comes on until air builds up to over 60 psi then this light and the buzzer goes off.
2. " P " parking brake indicator light comes on when you pull up the yellow valve knob for the parking brake.

Recently , the " P " indicator light is always on , even when yellow knob is pushed in.
When yellow knob is pushed in , the brakes do NOT grab , truck roles freely as it should.
Air system goes to 110 psi as it should
With parking brake OFF the brake canisters are operating as they should - fully retract into canisters
Brake fluid reservoir ... is full , this is for the hydraulic clutch I believe ??

Is there a relay somewhere that has gone bad causing this warning indicator light to stay on ?
The truck is operating normally aside from this warning light.

Any ideas ?