Water is pulled from the pool by your pool pump and pushed into the “PUMP” port of the multiport valve where it enters the filter tank. ... After passing through the filter media, the water exits the tank again into the multiport valve, leaving through the “RETURN” port, and back to your pool.
If the valves are older than 10 years you may want to replace the whole thing versus replacing just handles. Multiport valve manufacturers build their units to be rebuilt, and as long as the main internal parts are okay, the wear out o-rings and gaskets can be replaced.

Water is pulled from the pool by your pool pump and pushed into the “PUMP” port of the multiport valve where it enters the filter tank. ... After passing through the filter media, the water exits the tank again into the multiport valve, leaving through the “RETURN” port, and back to your pool.
If the valves are older than 10 years you may want to replace the whole thing versus replacing just handles. Multiport valve manufacturers build their units to be rebuilt, and as long as the main internal parts are okay, the wear out o-rings and gaskets can be replaced.