Golden Eagle 421
Flight Crew Commercial; Flight Crew Multiengine
Flight Crew Last 90 Days 125; Flight Crew Total 6300; Flight Crew Type 300
Aircraft Equipment Problem Less Severe; Ground Excursion Taxiway
General Maintenance Action
Aircraft
Before the plane was started; brakes and lines were inspected fully. Before start; brakes were applied and engine start was done with full brakes. When I first started taxiing the aircraft from the general aviation terminal; brakes were used to reposition the plane from parking to the taxiway effectively. [Later I] started applying brakes and noticed there was no brake pressure on either left or right brake. Did notice that the right engine low hydraulic pump light had come on. Engines were at idle. The plane veered to the right and the right main wheel left the taxiway surface. Tried using differential thrust from the right engine to raise the wheel back to the taxiway. The right main was not coming back to the taxiway surface. I kept the plane controlled using power as to not strike a propeller into a taxi light. Once clearing the propellers from striking a taxi light I tried again to bring the plane back to the taxi area using power from the right engine. The plane did make it back to the taxiway and the left main came onto the surface without an issue; still having no brakes. Once I realized there were still no brakes; I closed throttles; mixtures; and secured plane and then called the Tower asking for assistance.
Everything was fine on taxi. As we approached the departure end of [the] runway; the Right Hydraulic Low annunciator light illuminated. As soon as it did; we began to brake but there were no brakes at all. Immediately I informed the copilot that there were no brakes on the left side and he immediately depressed his brake pedals to find that there were no brakes on that side as well. Could not get directional control with rudder pedals. Engines were at idle. Right main gear left taxiway and entered grass. Immediately we veered to right and all wheels left taxiway into grass. Copilot attempted to steer back onto taxiway using differential power but grass was saturated from rain and was unable to do so. We did get left main and nose back to the pavement but right main got stuck in mud.
C421 pilots reported complete brake failure during taxi that resulted in a taxiway excursion.