It is a good idea to remove the hoses and tubing that connects the 3 main components (compressor, condenser & evaporator) - then blow the hoses/tubing and components. The oil will be far more effectively removed.
You can't blow through the compressor - unbolt it, and tip it over to drain the oil (rotate the internals if possible). This will get the bulk of the R-12 oil out.
Here is a 134-A conversion procedure written by someone with too much time on his hands:
No component changes are required to switch over from R12 to 134a. All the currently installed parts can remain and will work just fine with a 134a charge. While the system is open you will want to replace the filter drier and the expansion tube.
Couple of notes:
You will have to dump the remaining R12 before anything else is done. You can release it to atmosphere, but certified shops cannot.
They have special re-re (refrigerant recovery) equipment that captures the R12. When they have a full bottle it gets sold to a facility that purifies and repackages it for re-sale. The production of new R-12 was banned worldwide years ago, so the only source is what can be reclaimed.
Anyway, if a shop does this conversion, that recovery process will trigger an extra charge. If you do it yourself it is free.
Whatever way you do it, after the system gets dumped, it will need to be flushed, evacuated and recharged with 134a.
Flushing is required because the oil used with R-12 is non-compatible with the oil used with 134a. There is a specific, involved, expensive way to do this - or there is the easy way. Provided your system is not being changed over due to a "black death" casualty you can do it with all the components in place and using just compressed air.
Black death is just a term used to describe a refrigeration system that has been disabled due to a catastrophic failure of the compressor. All the components and connecting tubing/hoses are contaminated with burnt (caked on) oil and all the metal particles that resulted from the compressor failure.
Release all the tubing fittings that connect the 3 main components (compressor, condenser & evaporator). Remove the suction/discharge hose manifold from the back of the condenser. Pull the existing expansion tube from the tubing that leads into the evap. Remove the filter drier.
Using 100 psi compressed air at a good volume:
Blow the residual oil from the condenser and evaporator coils. Do this in in both directions. Repeat until you feel that the bulk of the old oil is removed.
Blow out the liquid line that connects the condenser to the evaporator.
Blow out both compressor hoses.
Understand that the old R-12 oil is not like "poison" to the newly added 134a - it just does not circulate as well. So if there is a bit that remains in the system it will not be an issue.
That's it for flushing.
Using new o-rings at the fittings, replace your filter drier and expansion tubing.
Add PAG 100 oil in quantities as below and close the system up.
•Compressor- (1 ounce)
•Evaporator - (3 ounces)
•Condenser - (1 ounce)
•Accumulator (2 ounces)
Evacuate, leak check (system should hold 28" Hg overnight) and charge with about 50 ounces (4 cans) of 134a. You will have to jump your low pressure cut-out switch to get the first 2 cans in. After that the system will have enough pressure to allow the low pressure switch to operate normally and keep the compressor running.
You will need to buy a set of adapters for the service ports so you can charge and attach your gauges - if you have them. The adapters just screw on to the existing service ports.
