310Psi head pressure is not too far off in a hot car at 100°F outside...
I bet you aren't getting much cooling at those temps though... do the normal things to improve things, clean the front condenser coil with chemical that will dissolve bugs, blast out ANY obstructions from the motor side of the coil, and verify it's totally totally clear, and all fins are straight... also verify the aux fan is working properly.
with a 134 conversion, you have to ignore the sight glass, it's for 12... ya can't put enough 134 in there to fill the sight glass unless its below 80°F outside...
if you want it to cool, you will have to replace EVERYTHING. Compressor, reciever, expansion valve and flush the lines, and replace the oil and recharge with R-$12!!!

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!