For Airborne Infantry grunt 75 . Dam big mouth full to say that.Iam just a crusty old prop pushin Jet suck blow and go Mech . easy for me to say.. Check your parking brake handle switch, its a spring leafed micro switch in the handle assy, Two wires run down the cable tunnel and out the cab , one goes to ground they other goes forward, You have three pressure switches (low Pressure) one on each tank , primary and secondary and one behind the trailer Air switch on the instrument panel (back side of the knob ). Make sure you don't have any breaks. disconnected wires, Jump in the Tech Data ( available here in the TM section FOR FREE. has the trouble algorythem in one of the volumes( 3 sticks in my mind) easy to find, There is an OHMS check described there, Have the same problem INTERESTINGLY enough , with the emergency brake handle up (engaged) if I put side pressure on it the light and buzzer go off! . have changed the switch, was intermittent. two pressure switches, were leaking and redone the grounds, STILL ON . Would appear the mode of operation in low pressure is TO GROUND to light light and sound buzzer Headed out of the country for a while , will work some more when I return, I was sidelined by a leaking air and water shutter stat . Good luck, and really , a lot of its in the Tech Manuals ( T.O, , technical Order for AF types )