The thing with R12 in your case is you only need a topping off , as you do not
have a large leak..
S o, that usually requires less than a lb.
Some will tell you that it has to be evacuated and recharged , but in cases
where the level never get low enough for the low side to pull a vacuum, there
is no fear of air having entered the system..
A proper 134a repalcemnt/conversion requires not only oil, fitting, and 134a,
but new expansion valve , high side switches, O rings , and flush...not cheap,
and after all that, you still wind up with a system
that can not handle the high thermal loads
Abd to be more specific, the system in your model was never a strong unit to
begin with...so , with marginal performance of 134a ., well you see where I am