its at 50 psi at 83 deg here and still doing the same kicking on and off rapidly.
You need to get your gauges hooked up to where you can read both low and high sides. I want to say it sounds like you are still low. But, too much can also cause low side pressure to go up. That chart posted in the first page is just a vague reference. A/C systems vary widely and there is no "set" pressure that they all will show when they are at the right charge.
When I KNOW that a system is low and I'm out in the field and not in the shop, I will just use the 12oz cans and an application hose. You can tell you are close to the right charge when it's acting proper, like good cooling inside for one, a sweating or frosty dryer, and a pump that is not cycling like crazy.
When I don't know how much is in the system, I have to use gauges. I read high side pressures as that is how I can tell what is really going on. Way high pressure means too much, low pressure on the high side usually means low on freon. Older systems usually don't run real high pressures either, which is partly why the GM pancake and hot dog compressors suck for pumping R134, they don't like the higher pressure that R134 usually runs.
Airflow is also really important in these old trucks. Most just reuse the old R12 condenser when converting to 134 and it doesn't have enough condensing for R134, combine that with the crappy old school metal fans and a worn fan clutch, the results are high vent temps and high low/high pressures and why older converted vehicles don't cool well at idle yet cool right down once the vehicle is up to speed for a few minutes with heavy wind flow through the condensor.
