The refrigerant R-134 does recycle through the system and and in the process is compressed, expanded, and changes from liquid to gas and back again. It's a thermodynamic process that uses many components in the A/C system. The refrigerant will leak if one of the seals is bad. Some leaks are gross, some real small. Your system has a leak. Refilling could be fine for a while, depending on the size of the leak. The A/C shops will inject a dye into the system. The dye is detected with a UV light. If the leak is small, it may take a few weeks, then they check for leaks. Once the source of the leak is found, it could be repaired. Keep in mind that the A/C shops have equipment to do the job right. This includes evacuation equip and lubricants that go into the system. They use a gage device to properly charge the system. They also "charge" the customer!I have never found a bargain shop for A/C. But when fixed right, it won't leak. Many cars go a long time without losing refrigerant.