Any time the system is open up the sytem will need to be vacumed down to remove moisture ect and to let new charge to enter freely ...to change over to r134 the system will need to be flushed ect...using a r12 comppressor might last awhile but will fail in the future ...r134 is a heavyer freon and r12 comp w2ill work for awhile ....need to replace orfice tube also
If sytem has failed due to comp going bad the ac accumulator needs to be replace also because of metal parts going thru the system .......
bg
November 2005