Vacuum pump question

[RedD250]

While diagnosing a vacuum leak, I tested all of my hoses, but never thought to test my vacuum pump for leaks. When the truck is runing, it's pulling a stead 28 in/hg, only fluctuates when I rev the engine. But when I turned the truck off and hooked my vacuum tester directly to the nipple off the pump on the truck, it won't hold vacuum when I pump it up. Is that normal operation?

 

[BigPapa]

Yes, there’s a check valve in the line coming off the pump up near the fire wall.