Clutch Piston Exhaust Blowoff Ball Valve
WARNING: This page is about a different car, the 2010 GMC Sierra and 2010 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Backfill fluid is constantly limited to a pressure of 2 psi by the action of the clutch piston exhaust blowoff ball valve. When fluid pressure in the backfill circuit begins to exceed 2 psi, the ball moves against spring pressure allowing the excess fluid pressure to exhaust into the sump.