Go back on the left side, and open the back shield,. Stand up on the step by the tool box. There is a gold block with a set screw with a lock nut around it on top of it. (it is pretty big)The set screw takes a allen wrench. Use a allen wrench to hold the set screw as you loosen the nut with a wrench.Then tighten the set screw a turn and tighten the nut. Go in the cab and check the pressure. If it is to high loosen the set screw. If it is still to low tighten it more. Do this till you get it where you want it.