3/8" Barb.

Maintain proper braking with one of these brake booster check valves, often located on the brake booster assembly. This one-way check valve allows the vacuum to build up in the brake booster and prevents the vacuum from leaving the assembly. This gives the brake booster a consistent supply of vacuum needed to assist the driver when braking. Made from ABS plastic.

Suits 18.75mm (0.738") hole, Check Valve push in bar is 15.50mm (0.610"), and the Barb is 9.52mm (3/8")