92 300D strange brake issues -booster or pump?

[pdrayton]

My power assist on the brakes has been acting up. Sometimes good, sometimes not.

Checked vacuum. Pump by itself does 23" HG. With all accessories connected there is 8" Hg at idle. There are no leaks - checked the VCV and shut off and the other one that goes to the cabin - they all hold vacuum.

The check valve on the booster line is fine.

However, the metal nipple on the vac pump (accessory connection) makes a buzzing noise and lets a vacuum through when applying vacuum with the mityvac. Should it do that?

The brake booster holds vacuum up to 20" hg (that's as much pumping on the mityvac my forearms could handle.)

Does that mean the booster is OK? Or could it be intermittent?

As far as I know, it's either the brake booster or the vac pump, but both seem to be behaving how they should. Or maybe the vac pump can't deliver as much as the booster needs?

Help! I need brakes...