My AC has been blowing hot air, did the recharge process didn't work. Now I can see some green fluid at the end of the low pressure line, and it's open. Had mechanic check it out and he said there's suppose to be a sight glass there and because there's not, I need to get a new low pressure line with one in it.

Is there supposed to be one and could that be why I'm blowing only warm/hot air?

 

The green fluid: This is put in the system to spot leaks. As for a sight glass, other than commercial trucks I have not seen an A/C sight glass it normally located on the receiver/drier.
The proper way to diagnose an A/C system is by first checking the refrigerant charge using a manifold gauge and noting the pressure on both gauges. This will confirm the refrigerant charge and diagnose blockages, such as a stuck expansion valve or clogged orifice tube. If the charge is adequate observe if the A/C compressor is engaging properly. Everything after that is generally electrical in nature.