When an A/C compressor fails, it flings metal shavings throughout the system. Simply installing a new compressor without cleaning the lines will lead to a repeat failure. An A/C flush gun solves this. You fill the steel canister with specialized A/C solvent, hook it up to your shop air line, and press the rubber cone nozzle into one end of a disconnected hose. Squeezing the trigger blasts the solvent through the line, flushing out burnt oil and metal grit.
Never flush components that contain filters or valves. You must bypass the compressor, expansion valve, and receiver-drier. Flush only the bare metal lines, the evaporator, and older parallel-flow condensers. After flushing, run dry nitrogen or compressed air through the lines to evaporate all leftover solvent before assembly.