One thing I should mention is that when I change
over a small system, (compressor burnout), I have
a couple of Q-size nitrogen cylinders and injectors
that I use to blow system flush through the tubing
and evaporator/condenser.

CY: Yes, that sounds wise. Get the acids out. Fortunately,
we're not at burnout, on this system. Yet.

If you get the right
compressor, you may not have a problem with the cap
tube when it comes to capacity if you go back with
R-134a.

CY: The system I'm servicing has a receiver. So, in theory
it should feed liquid The metering device is TXV, so it's
got some kentucky windage, as to delivering refrigerant.

I personally like to use R-416a which will
sub for both R-12 and R-134a. The R-416a runs a lower
head pressure than R-12 and will work with either
oil.

CY: When it came out, I got some 409a. Supposed to drop
right in, to sub for 12. Problem is, you have to deliver
liquid. Either pump it into the liquid side, or flash it to
vapor before it gets to the compressor. Which, 409a, is
more expensive than 416a, I find out later. Didn't know you
could sub 416a for both of those. 134a is a bit cheaper than
416, I think. I'd love to load the beast up with a couple
pounds of R-22, I still have a bunch of that. But, the
pressures are a bit too high.

It is recommended to use alkylbenzene or POE oil
if you have an oil return problem but I've never had
problem using 3GS mineral oil in a system with R-416a.

CY: Good to know. Thanks. Ideally I'd like something like
134a so I can use vapor to charge the system. No risk of
slugging the compressor.