thermal king sb3 sr+ won't cool

Hi, I'm a business owner that just purchased a reefer trailer from someone who told me it was working and actually showed me it was working when I came to inspect the trailer. After making the purchase and getting it back to my shop, I ran the unit over night and it did not cool anymore as it did when I went to inspect the unit. call a tech out and he said it was low on freon. charged the system with 7lbs of 404a and it ran great but the service tech said that something wasn't right about the system. it was reading 15psi on the low side and 300psi on the high but he couldn't see what's wrong so he put some dye into the system. well after about a day, temp started to go up and the system would not cool as it should. By the end of the week, the system was just blowing ambient air. called the service tech back out and he put a gauge on it, the reading was 1psi on low side and 200psi on high. he said most likely there's a leak. he looked inside and out with both the electrical and uv flash light but couldn't find any signs of any leaks at all. can anyone help me with this? anyone has gone through any problems like this, please point me in the right direction.
thanks in advance,
Mike

 

the unit has 4800 hours on it. the engine runs strong, oil filter was change last year (has 9/26/2016 written on it) there are normal oil residue all around the engine and compressor (old residue). the unit looks clean otherwise, doesn't look like they clean the unit at all to hide anything. my service tech I use doesn't work on thermal king reefer units so he's not familiar with the unit. He does knows his stuff when it come to commercial refrigeration, roof top units and what have you. one thing I notice was there were 2 sets of wire unplugged. one set was coming from what I think is a 3 way valve but not sure. the other set was cut and is just hanging there (looks more like a rip, not a clean cut) and I can not tell where those wires come from or go to. I have not plug the good one back together because I think maybe whoever unplug it left it unplug for a reason...

 

anyone?