Are you really working on a 25RCTF? That's one very old unit. GE 210 system if I remember right. OK so if you're facing the operator position the door to your left will have the brake fluid reservoir. If it's empty you have a leak somewhere. just for testing purposes, you could jump out that switch under the inner pedal connected to the master cylinder if the truck runs then replace that switch. I have worked on only one of these units in the last few years but back in the 1980's they were everywhere and I worked on them daily.