Before you decide that the problem is the pump, check the check valve. It is located on the pump where the main vacuum line attaches. If you remove the check valve, you can often see where is has failed (broken parts inside.) From your vacuum test, sounds like the check valve. The valve should allow air flow into the pump, but not back out. Good news is that you can remove the valve with the pump still installed on the engine. If it turns out to be the pump, you can usually overhaul it. You don't want to buy a new one.