I'd say the first thing you do is remove the hose from the vac pump to the
booster. (In the midst of this you might want to see about getting an engine
that sucks [literally, not figuratively] so you don't need that pump.)
Crank it up and see if the pump is pulling a vacuum. If you have a vacuum
gauge, so much the better, but if not you can get a fair idea by putting your
finger over the end of the hose. If it pulls pretty hard, it should be fine.
If your pump is sucking OK, you might see if there is a valve in the hose
that is not working right. Air should go through it toward the pump
(sometimes marked Engine----> on the valve) but not the other way.
If the pump and any valve are OK, look for leaks. The worst but most common
place would be in the booster itself, in which case you need a new or rebuilt
one. There is a place (name/address forgotten) in SoCal that will do it for
about $130.
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George Downs, Bartlesville, Oklahoma, Central US