I am having a problem with a Soft serve ice cream machine that runs too long. It has an automatic expansion valave which is set for 20 PSI. The machine was originally charged with 502... I have retrofitted it with HP 80, (R-402A). I weighed in the correct charge and the machine will not shut off. The ice cream comes out a degree or two above the recommended dispensing temperature. How can it keep running without freezing up solid?