A/C Diagnostic and Troubleshooting - Help Needed

I am trying to tackle the job of repairing my A/C system in my 2002 Ford Windstar. Here is the information I have gathered so far:

1st Scenario - Pressure Test with engine off, nothing running:

Low side pressure - 70psi
High side pressure - 70psi

2nd scenario - I then started the engine, rolled windows down, set rpms at about 1800, and turned A/C on full blast and let run for 5-10 minutes. Here are my readings:

Ambient Temp - 70f
Vent Temp - 68f
Low side pressure - slowly creeps up to 40psi, then compressor kicks on at 40 (for only one second) and pressure drops instantly to 20psi and compressor kicks back off
High side pressure - stayed around 140psi, then shot up to about 150psi when compressor kicked on (again, only for a second) then back down to 140psi when compressor kicks off

Note that the compressor only kicked on once every 10-15 seconds for only 1 or 2 seconds max

3rd Scenario - I "jumped" the low pressure switch forcing the compressor on and let run same as scenario 2 for about 10 mins, here are the readings:

Ambient Temp - 70f
Vent Temp - 70f
Low side pressure - negative 10psi
High side pressure - 210psi

My understanding is that if low side goes negative and high side is high, then the problem is a clogged evaporator core and possibly clogged expansion valve as well. Can anyone confirm this?

I am ruling out the compressor because it is clearly compressing or else it wouldn't be able to draw negative pressure on the low side.

Also, this van had rear A/C. Does it have a seperate rear evaporator core and expansion valve?

Any information would be greatly appreciated.

Thanks,

Nick

To me it reads as you are very low in refrigerant.

You've lost about 85% of your necessary refrigerant. Recover- fix leak and evac and recharge to factory spec- and then it should work.