Air carrier Captain reported that the parking brake was set during pushback and when the tug disconnected; the aircraft rolled forward.

Narrative

Pushed by ground crew and reached parking spot. Ground crew requested; 'set bakes.' I set the PARK BRK handle to On and saw pressure on the Brake and Accu Press indicator. I reported 'Brakes set; pressure normal.' The ground crew started to disconnect from the aircraft. The aircraft started to roll forward. I immediately used the brake pedals to stop the roll. Ground crew said; 'set your parking brake.' I cycled the PARK BRK handle from On to Off and back to On. I verified PARK BRK was on and confirmed accurate pressure on the Brake and Accu Press indicator. Parking brakes held pressure and the aircraft in parked position. Aircraft was disconnected with no further incident.