Coley

I found this all very helpfull as well, thank you all. I bought a truck which had just had speedo replaced but the replacement didn't work. Trying to solve problem myself and found one suspicious issue. One lead to the vss reads 12v dc current. Is this a problem? Thinking the guy who replaced it may have made a mistake. Anyone know? Thank you.

I have a '93 Mack RD that has a problem with the parking brake. The Yellow valve will not stay in. I took the supply line off the valve and there was very little air when the truck had full pressure. I traced the line to some sort of check valve that has 5 air lines going to it. Two of them come from the tanks and seem to have good pressure. Does anyone know what this valve is, or where the other lines lead to on this valve? It is located on the rear of the frame crossmember just aft of the cab on the driver's side.