power steering pump problems

[72 K20]

When I bought my K20 almost two years ago the steering sector seals were shot and the pump was shot from lack of oil. I rebuilt the sector and got a reman pump cartridge from napa since they couldn't get a complete pump with the return line in the right position. Got the pump together , filled it with fluid, started the engine, turned it back and forth a couple of times ,shut it off and refilled the reservoir , started the truck and had nothing.Removed the pump exchanged it at napa, that one grenaded internally upon startup,, literally broke the cartridge in pieces. Got the third pump and all was golden,,,,,,, until about a week ago. I noticed a growl while driving and thought possibly was low on fluid altho I have not seen any leaks. Checked the fluid ,, full,, hhmmmmmmm,, but foamy. This pump has worked great until now,, only about 3 thousand miles.

Called napa,, they said to call customer service. Called them , tech says I need to vacuum bleed the pump. I said What ?

He said to take an old reservoir cap, drill and tap for a hose barb and pull it down to 15 inches of vacuum with the motor idling, maintain that for 10-15 minutes and it would be cured. I asked why it would be messing up after almost 2 years of service.
He said that he couldn't explain that. He said that maybe it's from sitting. I told him that my other trucks don't do it and neither did this one until recently. He said again
I can't explain that.

I've replaced and installed at least a dozen GM power steering pumps over the years and have never had this problem whether it was a new or working used pump.

Have any of you experienced this ?

Thanks for any and all comments !!
Have a great day !! Tom