My pac break has been in the truck for 40k miles. I have only owned the truck for 3k miles so I don't know much about the system. Today the break just stopped working. So after messing with it I discovered that I could get it to work if I turned the switch on before starting the truck. With doing this I could turn the break off and on with no problems. But if you start the truck with switch off the break will not respond. Any ideas?

 

I have had all kind of problems with mine. pac brake was no help the second time I had a problem they just told me to buy new(with that advice a I would be purchasing from new company).
Since it sounds like the plate itself is moving freely it is most likely in the vacuum valve. I did as they said the first time and disassemble the whole thing and got it working. the last time I I just use the little button on the top and spayed wd 40( not the best fan of that but the lube an all seems to be less harmful of the o rings) on to the intake brass looking intake filter and did this many time while pressing the button on top and flipping the toggle switch and soon after the bake started functioning and has been working perfect. after you get it working keep using it so everything stays working even when you don't actually need it or I make a habit to use it after starting puts a load on the engine at idle. another story when the flapper got bound up with carbon. but hasn't missed a beat in a year now. I did buy there high temp lube that I oil up all the fitting on the brake