Had that happen on my last rig...took me forever to find out what it was, and the dealer didn't have a clue. Turned out, the plug on the master cylinder was loose (after I installed a compressor to the master cylinder bracket). I unplugged it, re-routed the wires, and clipped it back on, and all is well. Check that plug first. It takes about 20 seconds to register that it has been plugged in/unplugged, so when you plug it in, wait a bit.