SA-227 Captain reported a taxiway excursion during taxi out for takeoff.

Narrative

As Captain of the SA-227; there are specific tests pertaining to the nose wheel that must be done prior to flight. I was taxiing on Taxiway A at ZZZ; to the active Runway XX when; in the performance of my duties; the aircraft got away from me on this narrow taxiway and furthermore; my brakes had no act. Excitedly; I yelled for the First Officer to apply the brakes on his side. The aircraft did come to a standstill; but not before the nose wheel left the pavement and settled in the grass along side the taxiway. I called the airport manager on CTAF frequency and had him bring a tow bar to assist. Together with help from the office personnel and me; we pushed the aircraft back onto the taxiway. I inspected if any damage had been done; but there was none. So; we climbed back in and fired up. I finished my taxi checks on the actual Runway and then taxied back for departure without any further trouble. Going forward; if the taxiway is too narrow; I will be using the Runway for my taxi checks.