WM, here's my take on what's going on. First let me state that I am an ASE Certified Master Technician with over 15 years experience (including automotive HVAC systems).
First off, R134a (the new stuff) does not run the same pressures as R12 (the old stuff). Since R134a is not as effective, it must run at higher pressures with more cooling at the condenser to achieve the same results.
Secondly, you need LESS OF A CHARGE with R134a than you did with R12. If your system required 1.5lbs of R12, then after a conversion it should require about 1.1 to 1.2lbs of R134a.
Thirdly, the A/C compressor is capable of achieving a very high pressure ALMOST INSTANTLY. Half a block of driving with the A/C is more than enough to over-pressurize the system... ESPECIALLY IN VERY HOT TEMPERATURES AND ESPECIALLY WITH A MALFUNCTIONING CONDENSER COOLING FAN.
Now let me educate you a bit about the pressure relief valve. In the old days, cars' A/C systems used a mechanic 'blow-off' valve... this way if the pressure got too high for whatever reason, the valve would 'blow' to release the freon to keep the compressor, evaporator, lines, or condenser from literally 'blowing up'. When the EPA deemed R12 freon to be 'bad for the environment' and 'illegal to manufacture in the US past 1994' they also required that all new automotive A/C systems and all old A/C systems retro-fitted to R134a to have an electrical high pressure switch. This switch simply turns off the A/C compressor if the pressure gets too high, therefore eliminating the need for a mechanic blow-off valve.
Considering your system obviously had a 'blow off' valve, it may or may not have had an electrical high-pressure cutoff switch. Most high-dollar retrofit kits (like the ones the dealers special order) usually come with this electrical switch, and it is usually installed on to the original system (leaving the original blow-off valve intact as well) but this switch is usually calibrated to shut off the compressor long before the blow off valve would go off. The fact that your blow off valve DID go off tells me that there's a high chance that either this switch was not installed (as required).. or this switch was not wired in electrically.. or they used a switch that was incorrectly calibrated. If you did have an electrical cut off switch before, most likely it was calibrated for R12. Either way, it should have been verified to be functioning correctly before they released the car to you.
Likewise, if you had a malfunctioning condenser cooling fan, this should ALSO have been discovered before they released the car to you. I should also note that a lot of the high-dollar retrofit kids INCLUDE a larger condenser fan (or a higher volume fan), but this varies depending on the year, make, and model of the vehicle to be retrofitted. You need to find out exactly what was replaced during the retrofit service.
It's also possible that your blow-off valve simply failed due to age and that your A/C pressures never got high enough to cause a normally functioning blow-off valve to set, but since yours was bad it did. This is also a possibility as since the temperature was cooler when they performed the retrofit, then the pressures would be lower as well and the malfunctioning blow off valve would not have been a problem at that time. In this case it would be really hard to blame the repair shop (dealership) for this having happened... BUT if it were my shop and this all happened right after we performed those kinds of repairs to your vehicle, I would NOT HESITATE to fix this for you FOR FREE, regardless if I felt if was our fault or not... that is simple customer courtesy right there.
As for this being 'odd' or not.. I have to tell you from experience that these sorts of things happen more often than you would think, and the vast majority of the time it's really nobody's fault. Like I said, the temperatures were cooler when they retrofitted it, and also you had a (for the most part) malfunctioning system in the first place. They simply got the system up and running for you... but without it being a very warm day it's really hard to test the system to its limit. It's kind of like shooting the hull of a tank with one round from a handgun, and claiming it's strong enough to endure battlefield damage...
Regardless, I sure hope they work something out with you... maybe at least charge you for only the cost of parts.. that way they don't lose money and everyone is happy, but dealerships tend to be a bit 'less forgiving' when it comes to that sort of thing.
I will add this.. the $20 retrofit kits you get from Autozone and the like come with only a small can of refrigerant, a charging hose, and a couple of universal fittings. While this can be installed on many cars to get acceptable results, installing this kit alone on most R12 systems is probably not legal. It is up to the consumer that the appropriate parts are purchased (and properly installed) in order to have an EPA legal automotive HVAC system. Not trying to be a party-pooper here... I'm just trying to educate you guys who may or may not know better.
