What would be the reason(s) for the combo valve not pinching off the blow down hose?
Too large of a blast nozzle selected for the available air supply resulting in low system pressure while blasting. The consequence of low system pressure is a weak (insufficient) air signal pressure in the control line that forces the Combovalve to close.
From continued use the blast nozzle becomes worn resulting dropping the blast pressure and the same impact as above.
Faulty Deadman.
Defective Combovalve piston seal.
Blowdown hose worn by abrasive.
Obstruction blocking one or both twinline hoses. This is typically caused by abrasive backflow.
