Very good write up. A few notes to add to this.Ahem, may I share a lifetime's experiance in this area?
First.. the systems in most of these are very small.. at most I'd say they hold only a few ounces of Refrigerant. Keith said he was disconnecting the plate from the condenser. if his is like most there are "service valves" on the condenser. Most of the oil is in the comp. any oil in the plate would only be residual. really small compressors don't have drain ports. the only way to get old oil out is hardly worth the trouble but you would have cut the "process" tube off. turn the comp over to drain, then flush it with new compatable oil, shake it drain, a couple of times. pinch and braze closed the process tube. It's a mess and only worth doing on larger units.
Keith, is the new condenser R-134? the operating pressures should work on the old plate but I'd suggest you change it also. you may find that the old plate will only cool about 80% then it did with R-12 now if you do hook up the old plate you can use Liquid 134 (can upside down,) in the larger tube to reverse the flow back to the other service valve, then through the smaller. that should help get the residual out. but there will still be traces. service valves are designed isolate the condenser not the plate. (they can be confusing too)
Assuming you did all this the right. The thing to do after it's been running a day or two is to; yup! Isolate the comp cut the tube and drain and flush again.. also install a new filter dryer.
if you got lost halfway through the above.. better get a refrigeration tech. you do have a vacume pump, gauges and electronic scale to weigh in the right amount of 134? One problem that is very common with people that don't know they put way way too much refrigerant in the systems. Again,, if you stay with old plate, try to find the namplate on the system hopfully it will tell you how many ounces (not likely anywhere near pounds) the 134 should be about the same amount +- 10%
the new comp should have 134 already in it. it charges the lines when the service valves are opened, some have "quck connect self piercing" couplings. and no service valves.
134 is nasty too. caustic even..I think it eats fiberglass too..
I am a heavy Diesel tech and am a lic ac guy. So i have been here and done that so to say.
First , if the old system was R12 you must under stand that R12 was much more efficient of a freon with relative low "high" pressures. R134a is not and must run much higher pressure and still only deliver 80% of the cooling power.
R134a conversions are not recommended . Now that all of you are yelling at me i will explain..
A R134a system is 20% larger than a R12 to compensate for the higher pressures and system temp. In a conversion , you are going to expose a system to much higher heat and pressure than it was designed for... They don't have a good track record.. Not to mention that the system is 20 years old to begin with..
I will advise you to .. Full a very long vac on the system and re charge with freeze 12. You will not be disappointed! I have a 1990 Honda CRX " that my wife loves more than me" and it runs freeze 12 and puts out 39' air !!!!! with a head pressure of around 138 psi which is low....
I have done lots of tractor conversions and 134a is just not a good freon unless the system was designed for it. Freeze 12 does not care what oil you use so, even if you have a pag oil R134 compressor it will not matter.