Have you isolated the vac line from the vac pump to see how much vac the pump is pulling? Plug the end to the brake booster, and check the vac coming off the T in the line (if two T's, plug one). If you are getting around 22 'whatever units' of vac, then the pump is OK. I know you plugged the lines and ran vac to brake booster only, but this info would help you with a direction.
Oil in the lines and in the booster can sometimes be a rupture in the shutoff valve on the back of IP. Any oil dripping our of ignition assy? Bypassing all vacuum except brake booster would tend to make this unlikely, however if bad, the oil will migrate to brake booster, and can cause it to fail.

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John