The two speed rear is big and ugly looking, and standard equipment on the 6400.

Could it be someone switched the rear for a different one, possibly from a 1.5 ton with faster gears and a single speed? Then also used the two speed vacuum switch to interrupt the Hydrovac for some unknown reason?

I believe I have a folder with pictures of my 6400 before I started pulling it apart, same link below. Mine had two speed, all the vacuum lines and switch, and th Hydrovac, and I took lots of pictures.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-