Hi Bridget Bryant,
This happens when there is a clog in the system such as the expansion valve because the refrigerant is not flowing back into the compressor. A car may have an orifice tube as well, also causing not enough refrigerant returning to the compressor so it pulls into pressure retard mode (vacuum). Check out the images below for more explanation.
Thanks
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Tuesday, May 28th, 2019 AT 2:36 PM