A/C only cold after driving around for 5+ mins

I recently purchased a high mileage 4cyl for a new driver and want to make sure the a/c is solid before she starts driving it.

The a/c blows decently cold, around 60f after you drive around for about 4-5 minutes, but if just idling it seems to take forever. The compressor is on and I attempted to add freon. It said if the pressure was below 25 PSI the system is low. When I connected the hose it was reading 90-100 PSI with the engine off which is about perfect (matching ambient temp).

I started the car up to see if the pressure changes with the compressor engaged and while it blows ambient air it stays at around 90-100 PSI, but once you sit in the car and rev the engine a few minutes it will start blowing cold and the pressure comes down to around 45 PSI which should be almost perfect, maybe 5-10 PSI lower than charts say based on ambient temp.

 

Just to post an update I ended up throwing manifold gauge on:

I'm hovering around L: 45 psi H: 160 psi at about 90f ambient temp. This is after holding the engine at 2k rpm for about ~3 mins to get the compressor to actually kick in and see the low side pressure drop down. Getting about 60f out of the dash. I did try adding 134a, but did not make any difference as far as I can tell.

When I turn the a/c off the low side jumps up to about 60-70 and high side stays about the same, so it seems to hold pressure properly. I'm thinking it's just a weak compressor.

 

Certainly sounds like the compressor is failing if you have to rev it to get it to pump.