Date: 202108
Local time of day: 0601-1200
Location: PHL.Airport, PA
Flight conditions: VMC
Light: Daylight
Aircraft: PC-12
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower ZZZ
Aircraft component: Normal Brake System
Component problem: Malfunctioning
Reporter organization: Corporate
Reporter function: Captain
Reporter qualification: Flight Crew Commercial; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Communication Breakdown; Situational Awareness; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Maintenance; Ground Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Gate; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Procedure; Human Factors; Aircraft
Primary problem: Aircraft

Narrative: Cleared to taxi from FBO Ramp to Runway XX; follow proceeding Aircraft Y. After starting to turn left on ramp; noticed right brake was soft. Once straightened out; tried brakes again; right went down halfway and after pumping it was not quite back to normal. Transferred controls to FO (First Officer) to try his side. He stated his right brake was a little soft. Transferred controls back to CA (Captain) and I tried brakes again while taxiing to Runway XX. Brake went halfway down and after pumping 1-2 times; it was still soft. Got to hold-short line of Runway XX and right brake went more than halfway down.We were on Tower frequency and told Tower we needed to go back to FBO Ramp due to brake issue. We had two flights to airports with short runways; ZZZ1 and ZZZ2. I did not feel it was safe to proceed. We were cleared onto Runway XX and to taxi to Taxiway XX and then back to Ramp. Tower asked if we needed any other assistance. We passed negative. While on Runway XX; I tested brakes three more times. Each time it got progressively worse on right side. After exit on XX; I transferred controls to FO. His right brake went halfway down. We transferred controls back to CA and my right brake was still halfway to almost three quarters down to work. As I was making my final left turn into parking; my right brake went to the floor. I was at less than walking speed and was able to stop aircraft with left brake and beta. Subsequently shutdown without any other issue. FO escorted passenger into the FBO and I contacted maintenance; and operations. Additionally; I had noticed yesterday while picking up aircraft in ZZZ3 that [the] aircraft had [a] brake write-up [last month]. I went back through all the logs in [the] can and discovered that there were brake issues [a few months ago] as well. It appears that [the] brakes were having issues every 28-35 days from [a few months ago].




Synopsis: PC-12 Captain reported a gate return due to a partial brake system failure.