B200 pilot reported the wing contacted a hangar door while they were taxiing the aircraft.

Narrative

I was tasked to return Aircraft X to ZZZ after the landing gear had gone through a 3.5-day inspection at ZZZ1. Exterior preflight through engine start were completed without any abnormalities. As I pushed the power levers forward to conduct my auto-feather and propeller governor system checks; the aircraft unexpectedly rolled forward. The ramp area was extremely crowded; particularly around my parking location; so I chose to attempt to taxi to some clear space to better assess and verify the abnormality.Through the taxi; I become more and more confident that the brakes were not working properly. Variable degrees of negative prop pitch were required to control my forward; left; and right movements. I planned to return to my previous parking spot; shutdown; and report the issue to maintenance. As I came to a stop in my previous parking spot; the Director of Maintenance called and I answered before shutting down the engines. I advised him of the issues I was having. He said the brakes probably just needed to 'warm up' and asked that I continue to taxi around to see if things improved. I asked if he would join me through the test; but he declined.On my second taxi tour of the crowded ramp; I attempted to find enough space to taxi at a slightly higher rate of speed than my first in order to further solidify any abnormalities. Through this taxi test; I verified that the left brake produced nearly zero braking action and the right brake required excessive brake pedal pressure to be effective. Stopped in a safe space; I called to Director of Maintenance to give an update on the situation. He asked me to taxi between the last row of aircraft and the edge of the ramp marked by hangers and a fence to get as close as possible to the maintenance hangar. I questioned the safety of the taxi plan; but was convinced to proceed with caution.As I started to turn down the final taxi lane; I attempted to provide a little extra space between my right wingtip and the parked aircraft as I knew my left brake was ineffective making it more difficult to turn left than right. In my adjustments; I failed to effectively divide my attention between aircraft control; and wingtip clearances on both sides of the aircraft. I then felt the aircraft begin to jolt and applied firm brake pressure on both pedals until the aircraft came to a complete stop. Once I realized I impacted the hanger door with my left wingtip; I shut down the engines and exited the aircraft.