when i got the 51-720 series gmc rescue truck back in 2006, i was told that the brakes didnt work and the hydravac was the issue. since the M/C was bone dry i filled the system and bled all the wheels which took some time because due to the size of the truck each wheel had two wheel cylinders per wheel. i had a full hard pedal and very little stopping action. after i got the truck home i thought i had the issue corrected until i saw i blew a rear cylinder with all the fluid on the inside of the wheel. i was brake checking it and i put all the pressure i could on the pedal. after i disassembled the cylinders i found out why they didnt work. every wheel cylinder was frozen. how could this be? i had to use the pedal to blow the cups out. i dont know what it was but each cylinder had solidified with what ever the previous owner had used for fluid for the previous 55 years. i had to actually dig out the crap. after getting the cylinders lined i found out there was nothing wrong with the hydravac. maybe this is your problem
