Inline pilot check valves. Please see photos for details. These are larger & heavier than ordinary check valves. The valve in the photo is 1/4" Bspp size. It weighs 900grams.
***DISCLAIMER***
IMPROPER SELECTION, INSTALLATION, IMPLEMENTATION AND/OR OPERATION BY ANYONE OTHER THAN PERSONS WITH APPROPRIATE TECHNICAL AND MECHANICAL TRAINING CAN RESULT IN DAMAGE TO EQUIPMENT OR PROPERTY, SERIOUS PERSONAL INJURY OR DEATH. PROPER PRECAUTIONS AND SAFETY PROCEDURES SHOULD BE IMPLEMENTED PRIOR TO INSTALLATION AND OPERATION OF EQUIPMENT.