A privately registered Cessna 550 Citation Bravo aircraft, N4AT, was conducting a flight from Winnipeg International Airport, Canada to Bedford, Massachusetts, USA. After starting the engines on Apron II while under the guidance of a ground marshaller, N4AT began to taxi for the departure. During the initial turn from the parking area, aircraft control was lost and N4AT’s right wing collided with the nose gear of C-FXWT, a Bombardier CL-600-2B16 Challenger 605 aircraft, which was parked. The pilot of N4AT subsequently shutdown the aircraft and exited with the passenger without injuries. The right wing of N4AT was substantially damaged, causing a fuel leak on the apron. The left wing of C-FXWT was also damaged in the collision by the nose of N4AT. The TSB’s on-site examination of the aircraft and subsequent laboratory testing of the hydraulic motor and pump assembly of the hydraulic brake system revealed that it would not operate. On-site examination also revealed that the LO BRK PRESS annunciator light would not illuminate when the brake pressure was below 750 psi because the switch was in a constant open state due to corrosion caused by hydraulic fluid contamination.