FMM, do you have an Eaton or Timken rear axle? They are different in how they operate. My meager knowledge is based on the trucks I own and drive;
1) 1947 Diamond T model 509 with Eaton 2 spd rear end. Vacuum pulls differential into high range. Spring pressure brings it into low range. The truck dash plate and owner's manual instructs you to NOT park in high range because as the vacuum bleeds down the rear end may try to shift to low and there is a spot where it is in neither high or low. This fact, coupled with the parking brake on the driveshaft means the truck can roll if unattended.
2) 1950 Chevrolet COE with Timken rear axle. Vacuum pulls it either into high range or low range, depending on where shifter valve is set. It does not change as vacuum bleeds off. There is no mention of parking in a specific axle range in the owner's manual or other liturature. Parking brake connects to rear brake shoes by cables (much safer than the above system).
The Chevy has two vacuum hoses to the diaphragm, the Diamond T has one. Which do you have?
Kent
