Once the alarm stops (it'll be higher than the 75psi you mentioned previously), pump the brakes until it comes back on. I think you'll find it is the low air warning that always comes on when starting after sitting for a while. Hearing the alarm is good when starting... it just tells you to sit still until you're up to pressure. If you hear the alarm while driving, get off the road and stop.
2004 Damon Escaper LX400 4194
40' Diesel Pusher, Freightliner Chassis,
Cummins ISL 400hp