If the compressor is cycling very fast, usally indicates a low charge. This would also go along with no cold air from the vents. The X-terra's and Frontier's commonly have leaks from the flexible A/c hoses crimp fittings. You will usally see an oily residue near the leak.
Your First step is determine if you have a leak that requires you to add freon to the system. If so, repair the leak, the evacuate all air from the system and recharge the system with the proper amount. Then verify the systems operation and performance.
did you get the 55psi on the low side with the A/c on or off? If that was while the system was running thats too high. Do you have a set of gauges that you can test the high and low side at the same time? and if so what are both pressures?
I just bought a can of R134 which came with a gauge to attach to lo side and followed the instructions on the can, which said low side 55 psi when ambient temp = 95. A/C on when doing the test.
Since it ran cold for a while I am thinking there is a leak, but don't know how to check for leaks or bleed air out of the system.
Like I posted above, most leaks you can find with a visual inspection. Nissan's use a fluorescent dye in their R-134. Usually thats visible with the naked eye, but is really visible under a black light. The most common leaks are at the flexible hoses connected to the compressor near the crimp fittings.
I would recommend you take it to a shop that has the equipment to do A/c repairs. Because the system is pressurized , it can be dangerous if not handled properly. Most shops have a all-in-one machine that will recover and filter any residual refrigerant, evacuate and air or moisture from the repaired system, and then using a scale will refill the precise proper amount back into the system.
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