[michaelovitch]

You need to check the whole system to see if the is any leaks.

the pump is not very often the problem

the servo (brake booster/mastervac) the big mushroom behind the master cylinder can leak....

you can test it with smoke (smoke machine or cigar) you fill it directly with a hose in its own hole.

the hoses between it and the pump too can crack and leak

don't forget the whole hvac system works with vacuum too...if a hose is leaky, no more enough vacuum.

condamn the tiny hose of the hvac by pinching it
then test the servo plugged directly on the pump.

you can put the finger on the nipple of the pump too it should blow job your finger.

you have to eliminate the potential circuits one by one to find the problem.