So... Yeah. The stuff is basically propane/LPG, more or less. EPA doesn't like it, but it works just fine in terms of making things cold on one side and hot on the other.
Propane(and things like it - butane, isobutane, a mix etc) is a liquid at a certain temperature and pressure. Add more temperature or less pressure and it turns into a gas. Cool it or compress it and it becomes a liquid again.
This is basically how an AC works, generally speaking.
The thing to note, though, is that once you hit the 'vapor pressure', the pressure won't keep increasing as you add refridgerant, because it will simply turn into liquid.
In your case, that's what happened - you ended up with a lot of liquid in the system.
My guess is that it settled in one spot over the winter, and when you turned it on, the compressor pumped a lot of liquid through the system all at once, and created a really high pressure which blew the relief valve.
I'd aim for a lower low-side PSI - 25-30 should do better, depending on temperature.
Also, as a general rule, you should try to evacuate the air out if you can, if only to prevent an explosive mixture of propane/hydrocarbons and air.
No, you probably won't have enough air to actually make it explode, but it's still a good idea. It'll probably make the system work better, too, as air won't compress to a liquid. And it's the gas->liquid "phase change" that moves a lot of energy in an AC system.
Also, you commented on the AC stopped blowing much air? That's because the AC evaporator froze up. Propane at those pressures in a R12 system will produce plenty cold temperatures, and it's more efficient at moving heat than the R12 that it replaced. This simply means that you'll end up freezing up more, unless you lower the low-side pressures.
You can also get one of those "therm-O-disk" 32F thermostat switches and just stick it there on the evaporator line, then wire it inline with the low-pressure switch next to it. Once it gets cold enough, the thermostat opens up, which stops the compressor and lets the system warm up again. You won't notice the difference inside the cab, as the condenser is still near freezing.