Do you know if the acquaintance replaced all the seals in the system? This would be necessary in a 12-to-134 conversion. As a first pass, sounds like you have a leaky system...

And, I've never had to fill refrigerant from the high side on a properly-working system - yikes, don't even like the sound of it!

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'83 300 SD
'05 E320 4Matic
'06 BMW 530 xi
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr