[T77911S]

i need some help on a r22 5 ton unit here at work.
we have 2 roof top units that are not very old. one has never worked since it was installed. ( dont even go there. it was not my doing and i had no say so about it) BTW, TRANE installed the system.
here is what is going on. the unit is freezing up. but, it freezes on the low side from the comp to the service port just outside the unit. it looks like it starts at the comp, and works its way to the port, but once it starts to freeze, the whole line kinda freezes at once. the port itsefl does not freeze and it does not quite freeze all the way to the port unless it runs long enough. low side is about 48-50psi and hi is about 190. it has a site glass and looking at the glass it looks low. i did charge it some but i did not see any improvement, plus, trane has been back out here and i am sure they would have added r22 if needed. i dont know that if the compressor was bad, would it make it look low according to the site glass?
my thought is a compressor, i just dont have that 100% sure feeling and i dont want to commit to it just yet.
i know the trane guy well and he is a good guy. if all it is is that it is low, he would have fixed it. when trane installed it, they worked on it for almost a week i think, but it has never worked.