I do not recall the details of the G-749 electric parking brake, but I expect
it to be similar to the one used on the G-744, G-742, and "G whatever a M-37
is" series trucks (when so equipped). These all release automatically when
you step on the service brake. They also work wonderfully.
As Joe Shannon pointed out, the body of the master cylinder looks identical
to the one use on the G-742 series (I even think they have the same number in
the casting), but they are different internally, and cannot be interchanged
without experiencing either (a) no brakes or (b) brakes hold but don't
release.
I had a problem with my M-45 similar to what you describe. After years of
working perfectly, when the truck came out of winter lay-up last spring the
brakes wouldn't completely release after application UNLESS there was a sharp
application and release. Rebuilding the air-hydraulic cylinder corrected
this.
Hope this helps,
David Doyle
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