As a general matter of pricipals I would go ahead and rebuild both the "Selectair" valve, and the shift cylinders in on the transmission top cover. I don't know what year of truck yours' is but the shift cylinders rebuild the same. The earlier transmissions had to have the main fork adjusted for the neutral position, the latter ones did not. If your truck is losing air pressure quite rapidly when the engine is not running, spray the shifter valve, and all fittings, regulators, cylinders etc. looking for a leak. Most times when the shift cylinders are leaking internally they dump air into the transmission case. If you remove one bolt that retains the top cover of the transmission and listen closely, you can hear it. Spray soap on the missing bolt hole and look for bubbles. There should be three regulators on the top of the transmission also. I think their output pressures are about 60-65 psi and this is adequate to operate the trans in any weather. Both Mack, and PAI have rebuild kits put together. The labor only takes a couple of hours to accomplish once the center floorpan is removed from the truck. There are too many small parts to monkey with to not go with a pre packaged kit. I think mine were about 70.00 for both kits and a transmission top cover gasket. When you've got the trans apart, look at the shifting fork for wear/damage. That SOB is expensive but it's not a high wear item from what I've seen. Rob