Yes, that's a pretty significant leak. Since it is a converted 134 system, get a can of 134 with UV dye. Read the fine print and make SURE that the dye has Ester oil rather than PAG oil. PAG should not be used in converted systems.

Once the dye is in with enough refrigerant to cool for a little while, use a black light to look for the leak. If you don't find it at any of the joints or the normal places, look at the condensation drain water. If there is UV dye in the drain water, then the evaporator is leaking.

Good luck,