I don't know why GM used the vacuum 2 speed in their 2 tons. The electric 2 speed used by 450 and bigger GMC and Ford and International was so much better. Vacuum leaks were always a problem. Either you would get stuck in low with not enough vacuum to get to high or if you were in high and lost vacuum, it would try to go to low the first time you let off the gas. If you didn't give it gas to complete the shift it will grind and grab and sound like the whole rear is torn up.
Down shifts are completed by the drive shaft speeding up. A dead truck can be shifted into low by putting transmission in neutral and turning the drive shaft until she ratchets into low. Up shifts are completed by the drive shaft slowing down. This can be completed in a dead truck by jacking up one rear wheel and turning it until it ratchets into high range. There were several ratios available for the the 1350 axles. The ratios were pretty good but the vacuum shift was crap. 5.14/7.15,5.83/810, 6.33/8.81. 1954 calls it a 13600 with either 6.33/8.81 or 5.83/8.10
Tim, you know I am of the generation that tries to fix it and then reads the manual if we can't fix it. I think it is saying that a leaky diaphragm leaves you stuck in low. Back when these things were pretty new, we usually checked to see if we were getting vacuum to the chamber first because chambers usually weren't old enough to fail yet. Now after 60 years, very few chambers still work.
