I can charge the air on my 99 Cherokee sport and it works great but leaks off after about a week. I took it to the mechanic and, after using dye, he can't find a leak anywhere. Could it be leaking in the compressor? Behind the dash?

 

It could be leaking at the *cooling core*? Basically the heater core for the AC system which is in the airbox behind the dash. This means removing the hvac box and disassembling it. Not cheap to have a mechanic shop do this but if your handy with tools and a do it yourselfer there are plenty of videos online tutorial this without completely removing the dashboard.

 

On the firewall, passenger side near the bottom, is a drain hose for the hvac box to allow condensation from the evaporator to drain out. Look down there with your UV light. Also check under the dash behind the glove box for any signs of dye.