So what happens when you mix DOT4 and DOT5 fluids? The Air Force crew cab thread indicated that the military uses DOT5 fluid in at least some of their vehicles. I have an Air Force crew cab with a chronically spongy brake pedal since I added longer braided hoses front and rear. Until now I just figured my brake bleeding skills sucked as I've attempted to bleed the system many times using gravity, pressure and vaccum methods (always with DOT4). Could this be my problem?