75 pounds, if your gauges are accurate is an empty system. I don't know exactly what you jumpered, but I fully expect the system is virtually empty. You may have other problems as well, but your pressures should be over 80 static, since the compressor is not kicking in, this is a static measurement.

Anything that says "tune up in a can" is most likely snake oil, regardless of what system you are supposed to put it in.

I strongly recommend that you don't go further down the snake oil path and use alternate refrigerants. Your system was designed for R12 and the 124 a/c systems only have marginal capacity. There are disadvantages to all the alternative refrigerants.

To fix this system, you need to find the leak, COMPLETELY flush the system, replace the filter drier, put in the correct amount of mineral oil, thoroughly evacuate the system and charge it with R12. R12 is going down in price with every passing day and even at $29 a can it will be a small percentage of the overall cost of fixing your a/c system.

Best of luck,