The low pressure switch is to protect the compressor, it can be any where on the low pressure side, since the compressor is running it is probably okay, if the compressor is cycling on and off, then you are probably too low on freon still. Did you vacuum down the system before installing the freon, if not then there is air in the system and it will probably never get cold, it has to be vacuumed to around -29" hg (mercury) not quite a perfect vacuum but, close, then leave it there or continue to vacuum it for about 15-30 minutes, this boils the moisture out of the system, which is also important, moisture and freon form acid in the system, that's why it has a dryer in it. If there is a high pressure switch it is usually on the receiver dryer or by the compressor, it is also designed if pressure is too low, to shut off the compressor clutch, since the clutch is engaged and staying engaged, you may or may not have enough freon or too much. Did you put the correct amount of compatible oil in the compressor (for R134a), did you change the 'O' rings to the ones compatible with R134a? If the system is working properly, the high side of the compressor to the condensor should be so hot you can not touch it for very long, if you feel by the expansion valve it should be freezing cold on the side that sprays into the evaporator, and the evaporator should be cold across all the coils. If you have pressure gauges that will tell alot about what is happening, but not everthing. Post your results. I don't know what this barrier is you are talking about. Good Luck.