I’ll give some history.
This fall, my AC was working well.
This winter, I replaced my compressor because I was getting startup issues.
Last week, I recharged my AC. It seemed to be working well, but today, a hot day, I know it’s not working properly.
Today, I vacuumed the system and recharged it, but it still is not blowing as cold as it should.
What could be the issue? I didn’t personally change the compressor, so it’s possible that it was not done properly. What should I check to verify?

 

If your high side was high (over 300 psi) and the low side too low it means you have a restriction in the system - most likely the accumulator. Replace it, vacuum it again - a couple of hours at least - and confirm that you have no leaks then recharge.

 

You got me with " I changed the compressor because of start up issues ", curious as to what start up problems would a new compressor solve. Since you didn't do the swap yourself, who is pulling the vacuum and who is charging the system, you or someone else. How long after pulling the vacuum does it hold before you start gassing, 30 minutes or more. Unless it holds as close to -28 as your pump can go, for minimum of 20 minutes , you may be having a leak. As for charging, I assume you are recharging with 134a. It's hot out there now and you need some extra cooling while charging. You should have both fans on when charging and I add a small trickle with the water hose spraying directly on the front condensing coil. That helps much to pull out the heat as you gas. Now, what are you gassing up to on the low side, I think with 134a, you should be around 35-38 on the low side with 134a gauges. Now if you only gas to these figures at IDLE, you'll be under gassed. You need to rev it up to 1500 -2000 ball park and ensure your low side is above 28 or the evaporation coil will freeze up at lower rpms and block air flow. I am not an expert but have fixed quite a few and these are things I learned.

 

The engine would either idle at hight RPM or just shut off while driving. There were also some engine codes I don't remember. When the compressor was replaced, it was noticed that the old compressor had a lot of resistance in its pulley wheel.
A couple of friends of mine who, while mechanically inclined, are not mechanics changed the compressor. It's definitely possible that they messed it up. I've recharged before, so I felt confident in doing it myself. I had it vacuumed by a separate mechanic.
Not sure. I was not familiar with the vacuum process at the time, so I let the mechanic handle it and he said it was good.
I've actually been using 12a. That's what my local parts store told me to use over a year ago when I needed to recharge and it worked. Is that a problem?
Noted!
That's definitely the range I was aiming for, but it still wasn't blowing cool. I added until the compressor started clicking off presumably due to overcharging then let some out till it stopped. It settled around 60 PSI a little cooler, but still not as cold as it's supposed to be.

I've bought a new accumulator as per pscarbor's suggestion. I'll install it myself. Though, I didn't measure the high side pressure because I don't have a tool to do that and I didn't want to go back to the mechanic who vacuumed it because it seemed that the only AC repair he was familiar with was recharging and I clearly wasn't simply undercharged. He also thought my new cans of a12 were empty when I knew they were full cuz I just bought them and they clearly felt heavier so he wasn't worth my time.

 

If you don't have the right tool for this (or any) job STOP! You need to read high side pressure in addition to low side, otherwise you run the risk of overcharging which will probably destroy your compressor. Go buy some gauges, they're aroiund a hundred bucks on Amazon; or better still, don't mess with the A/C if you don't understand it.