1997 Chevy Tahoe A/C

The fact that you had to recharge it once indicates it has a leak. The time span until it happened again indicates it's a pretty good size leak. It needs to be leak tested to find the leak and repair it.

You need to just find a different shop. This is a clear, colorless, odorless, pressurized gas. You need either a very high grade electronic leak detector or add U/V dye to the system and look for a leak with a U/V light and glasses. This requires a real talent and a lot of experience but there is no question the leak is there, they just aren't smart enough to find it. This is a sealed system. A leak is the only way for the refrigerant to come out. Some components are very hard to access and leak check also so you need someone that specializes in or does a real lot of A/C work to find it. DO NOT use any of those snake oil sealers sold. All they do is plug up the system and everybodies equipment.

You find a different shop. It obviously IS leaking and you just need someone that has experience in finding them. It should be that difficult. It's a good size leak.

You say they don't see a leak. Has anyone offered to pump down the system and check it for leaks? This is a common problem and is usually the connection under the air box in my experience. I don't know what it is about the location of this junction, but the O-rings in this connection are proned to failure and leaking. It is the number one leak I find in these vehicles including my own 1999. The system still needs to be pumped down and emptied of refrigerant before opening the system and it would pay to have a complete O-ring/seal kit installed if the budget allows it. I agree tho', no snake oil leak sealers. They don't work and gum up expensive equipment.