vacuum pump failing? pulley slipping

[ff43sc]

recently I have noticed my vacuum pump pulley is slipping when i first start my truck and the air conditioning is on. (90+ degree outside temp currently) The serp. belt was replaced to rule a loose belt out. And belt tensioner was replaced last year. The alternator and all other accessories don't seem to slip, making me think the vacuum pump is failing. This only happens occasionally and if i turn the ac off, the slippage stops. Is there a way to add oil to the vacuum pump, I saw a small bolt/metal cap area on the back of another vacuum pump i have, and wondered if i can simply squirt oil in that hole to see if a lack of lubrication is a cause of the pump not spinning as it should.

I haven't tried spinning the pump with the belt off yet, and it still makes plenty of vacuum so appears to be working fine, except for the occasional slippage..the pulley appears clean...

[DaveBr]

Your vac pump might be slipping but I would check your AC unit. Sounds like it's tightening up on you. After some more running it will seize.

[rustyk]

I agree. Vacuum pumps rarely fail by seizing, just quit sucking like they should.

[DmaxMaverick]

Check your serpentine belt routing. Just because it "fits", doesn't mean it is right. On a few models, including many 95's, the belt can be installed wrong and it seems right. When this happens, the belt can be too loose. The tensioner should require just about all its throw to get the belt on. If the tensioner is marginally taking up the slack, the routing is wrong, or the belt is too long.