The trigger valve works in conjunction with the top valve to trigger your nail when the trigger valve is pressed, it creates a difference in air pressure between the air reservoir and the cylinder, which causes the top valve to open, most triggering the wrench valve. A common problem is a failed o-ring on the inside, when this happens, air may leak from the trigger and the will not fire normally, replacing the trigger valve assembly has restored Compatible models: Fits for following models: 1/ 2, D51850 3 1/2, D51276 15, D51257 16, D51275 Type 1, D51855 Type 1, D51321 Type 2, D51822 Type 4, D51844 Type 4, D51257K Type 1/2 16 GA Finish Nailer, D51276K (Type 1) 15 Gauge 1inch to 2-1/2inch Finish Nailer, D51238K (Type 1) 2inch 18 Ga Brad Nailer Install: 1. Remove the pins from the nail 2. Pull open and remove the trigger valve 3. Apply a small amount of grease on the O-ring, insert it to fix it, and check it Colour:black Material:Plastic + Rubber Package Contents: 1 set x Trigger Valve Assembly Only the above package content, other are not included. Note: Light reflection and different displays may cause the color of the item in the a little different from the real thing. The measurement allowed error is +/- 1-3cm.