AutoZone loan gauge sets. The connectors are under the hood and under the front grill.

When mine made a hissing sound it was accompanied by a slight sweet smell (that was the oil). If you have a leak you can add dye, there are two drain tubes that go from the air handling box (the evaporator sits inside this box, the box is inside the dash) and drain to the outside of the car above the transmission. If your evaporator is bad you'll see the dye coming out of those drains.

Good luck!

Peter