Ford A/C :: Compressor Was Cycling And It Never Engaged

My 2001 Expedition 5.4L sat for 2+ years and due to a new child, I put it back on the road late last fall. Last week, we had our 1st "must use AC" day and it didn't work.

I listened to see if the compressor was cycling and it never engaged. I picked up a gauge and r134a kit.

Outside temp was about 60F, I think my therm was showing 57F to be exact. When I checked the low side pressure with the engine off, the gauge reading was below 15. It made sense that the compressor wouldn't start at the pressure. I put in about a 1/2 can with the motor running A/C on max and it was showing 40, but still no compressor cycling. I then shut the motor down, let it sit for about 3 mins and restarted. This time, the compressor started to cycle, it was cycling quickly though, I'd say about every 3 seconds.

With the compressor cycling, the pressure would jump from 17 to 40. With the motor off, it would read 50. Still no cold air. I put in about 14oz of R134a.

I guess what I need to know is if I should I keep adding R134a until it stabilizes around 40, even when the compressor is cycling? Also, where should the static pressure be? It's sitting at 50 now. I've been told it should be around 80, but I'm thinking that might be dependent on outside temp.

BTW, I'm now thinking that the reason that I'm not getting cold air and the compressor keeps cycling is because I need to add a lot more R-134a. I think when the compressor starts and draws coolant, the low side pressure drops to low, causing the compressor to turn off. When the compressor cycles off, it sees the low side pressure at 40 and starts it up again causing the coolant draw and low side pressure to drop to low again. Am I right on this thought? It looks like I'll need to add another can to get the pressure to stay above 22 when the compressor is running.

One other thing... How much R-134a the system takes? I need to know because I have access to a vacuum.