Before condemning the EGR valve because it doesn't hold vacuum, you must first determine if it is a ported EGR, or a Positive back pressure(PBP) EGR. A PBP valve will NOT hold vacuum unless the engine is running at a rpm higher than idle. It needs a little backpressure from the exhaust to seat a valve inside, before the vacuum can control the operation of the orifice. I would suggest testing it that way before deciding it is bad. Not all '75 engines had them, but they were available, so....
Most of the time, a PBP EGR valve will identified with a "POS" stamp on them....you might look and see if yours has that. Regardless of which type of valve you have, you will need to replace it with the same type, if you do replace it.
I can e-mail you a PDF on identifying and testing the different valves, if you want.
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Joel Adams
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