My F350 7.3L diesel brake light came on and the brakes became hard, like the booster or vacuum pump failed. Belt looks good on the pump and the heat/ac ducts are changing so I believe the pump is working. How do I check the booster? Thanks in advance.

 

Engine off, pump brake pedal a few times to use up reservoir vacuum , hold pedal down and start engine, pedal should sink down some as vacuum is built up. Booster failures rare compared to vacuum pump failure..

Some diesels have hydroboost and utilize power steering pump for power brake assist, vacuum is for HVAC controls.

 

92 would have vacuum pump. Common for a failure given it age.