Quote Originally Posted by lana
I could not understand why the discharge pressure came to vacuum? Also, there is a check valve on the discharge line but I could not see where the pressure gauge was connected, before or after the CV
lana, if the liquid line solenoid was de-energized then there would not be any refrigerant to create a suction pressure, hence the vacuum. At the same time, if the compressor is running and there is no refrigerant, the discharge pressure will not increase. In other words, the compressor would just be running in a vacuum.

As you pointed out the discharge pressure gauge could be upstream of the check valve, so the discharge pressure could have indeed read a vacuum also.

I cannot go for the thermosyphon theory. If the liquid line solenoid was open, the liquid could flow into the evaporator IF the evaporator was colder to allow migration (another term that could be used incorrectly for a thermosyphon). A thermosyphon effect/flow is created by a difference in densities of the volatile fluid.
The fluid density difference is created by a temperature differential between two points. Then the fluid will then actually circulate.

However, you said the system had been off several months so I cannot see the refrigerant migrating to the evaporator. The evaporator was probably just as warm as the receiver.

In all probability, the problem was caused by the defective? thermostat not allowing the liquid line solenoid to open.