Here are 2 types of Chlorine feeders:
“off line” type of feeder
The off line feeders need periodic maintenance and replacement of feeder tubes, fittings, and valves but can be modified to work on systems with inlet air leakage. This is the unit I prefer for chlorine pools.
“inline” type of feeder
The inline feeders do not work well on older systems where some air leaks into the inlet side of you filtration system as the air ends up in the feeder defeating its operation. It only takes a very small air leak to stop this device from operating correctly. Often there is no easy fix as finding and repairing a small inlet leak may be cost prohibitive.