im sorry but im licenced in air condition service, and some of what has been said is a little disturbing
you must be very carefull on giving advice when you dont know what the law is first and the procedures are in service and fitting .....
first of all yes there is a flush we use to clean out all contaminates ..... you cant use this through the compressor !!! small alloy particles build up in the system due to the compressor wearing out over its use .... the receiver drier is a filter but if there is grime in the system after the drier it can block the TX valve, you need to flush!
nitro is used to test for leaks under high pressure 400psi or there abouts its not liquid nitrogen!! its just an inert gas used to find leaks.... vacuum is used to just take out air so all that is put in the system is R134a not mixed with air or any other gas
R134a can be used in any car if its done right!!! its known as retro fitting for R12 systems
the pressures used for R134a are diffrent so pressure switch's will need to be changed for starters... an a/c system blows up cause someone had no f'in idea what they where doing
like the guys out the back of wreckers filling systems up with LPG!!!!!!!!!! mmm
just make sure you get the right advise