Is it R12 or 134A? Short cycling is a sign of low gas. If the charge is good?, then try hitting the accumulator. Sometimes after sitting all winter the screen clogs. Be gental, use the handle part of a hammer. (Accumulator, Big, alluminum thing that the lines go in near the compressor).Make sure car is running with ac on high when hitting it. Also check all the connections. If there is oily stuff near/on a connection then its leaked. common places are at the comp, where both lines enter. the lines near the comp. and the lines at the evaporator. Also evapotator may have let go. @$360 for my wifes car last week. Ouch.
is there pressure when it is off?. BTW, Lines seldom clog, only accumilater due to filter. If there no pressure when its off, your out of gas. IE you have a leak. The reading you get is the compressor compressing air.
In order to find the source of a leak, the technician will need to add refrigerant to the system if only a small amount of refrigerant remains, and the refrigerant is at a pressure of less than 50 pounds per square inch. However, if there are at least a few ounces of refrigerant already in the system, a technician should not need to add refrigerant in order to identify which components will require replacement. (Keep in mind that a greater quantity of refrigerant -- usually a minimum of 1-1.5 pounds -- must be in the system in order to determine if the system will provide cooling.)