Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Distraction
Aircraft Equipment Problem Critical; Ground Event / Encounter Loss Of Aircraft Control; Ground Event / Encounter Aircraft
Aircraft Aircraft Damaged; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft
The aircraft was being taxied and then blocked in at position [parking spot] on the company ramp. The APU was not used after landing but the ground power was connected and powering the aircraft. The Captain began the shutdown flow as well as the normal shutdown checklist while I contact company ground reporting our landing 'on' and taxi 'in' times. The Marshaller signaled to the Captain to keep the brakes on and wait for the aircraft to be chalked in which he complied. I then noticed that the aircraft was moving backwards and immediately notified the Captain. The Captain and myself immediately pressed on the brake pedals but the aircraft continued to roll backwards. The ground power unit then disconnected from the aircraft and the Captain immediately switched to battery power and started the APU as well as the right hydraulic pump. The Captain then turned the nose tiller to help prevent contacting an aircraft behind us. The aircraft contacted another aircraft parked at position [different parking spot]. We then contacted company ramp as well as our company maintenance control as well as our flight control.
[Report narrative contained no additional information.]
Flight crew reported a loss of aircraft brakes while parking that resulted in a collision with another aircraft.