Hi there,
Here's another thought for you to consider... it doesn't take as much R134 as it does R12 to fill the system, and high low side pressure is a classic indication of an overcharge. It actually only takes around 2/3 the charge in an R134 system by weight. If your mechanic is trying to put the same amount of R134 by weight in the system as was called for in R12, it's overcharged. I recently converted my system - it's a hybrid, having a GM compressor that ran R134 to begin with and the Mercedes 300D A/C system, but it works fine. It only took 2 cans of R134 to fill the system. By the way, if the original compressor had bad valves, it would show up as low high side pressure...
Richard Wooldridge
'82 300D/4.3L V6
'74 280C
'77 XJ6L
'77 280Z