A spring by-pass valve is used to regulate water bypass around filter and eliminate high filter pressure on systems with 2 speed pumps. Valves open at 5 lbs and is fully open at 17 lbs.
 

DelCheck valve can be used as a bypass or as a regular check valve.
1-1/2" Slip or 2" Spigot, 5 lb spring, PVC
This bypass valve is not the one used in Del's ozone injector manifolds.
According to manufacturer, they do not sell the bypass they use.

Use for air blower installations, spring loaded, 1/4 lb. spring for a minimum restriction of air flow
which results in less burden on the blower motor.
 

Spring air check valve, 1-1/2" Slip or 2" Spigot,
1/4 lb spring, white body, PVC

Del 1/4" barbed ozone check valve, one piece, spring loaded

Rainbow spring check valve, 1/4" tube compression,
corrosion resistant for chemical feed

Rainbow spring check valve, 1/2" tube compression,
corrosion resistant for chemical feed

Rainbow  1-1/2" Slip or 2" Spigot, 2 lb spring,
white body, used with some Rainbow chemical feeders, corrosion resistant