Ooo, that's pretty low for that 300# high side pressure. Now it's down to cooling fan functionality or restriction in the system. Has this system been open/empty for a while? Cooling of the condenser (in front of radiator) is essential for head pressure. I dumped my fan and clutch for thermo-electric, gave me more engine power (and a lot less noise)and colder A/C on a retro 134A in an '88 Troop. After a good evac and charge, even a saturated drier will cool for a few months.
Hard to believe the exp valve, 30 years of 911 Porche's with the same generic valve and I've replaced one, on a car 25 yrs old. Maybe Zuzu is different. Good evac and good ester oil usually cleans em as new. Keep postin...Cal