Originally Posted by
SRXSRULE
Ive worked on a few of these trucks, even the ones with the 350 chevy.
Its a vacuum pump, nothing more. Has an oil pressure feed line and oil return line.
The Alternator driven vac pump is used for the power brake booster, exhaust brake if equipped, and various vacuum operated valves.
One more thing about these trucks with the chevy motor. If you live or drive much in the mountains its very easy to run them out of oil and blow up a motor. This is an engine braking deal and burning oil. I've never thought about it before but that vacuum pump could be the source of burning oil in a high vacuum condition during down hill engine braking.