Date: 201511
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Parking Brake
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Situational Awareness; Distraction; Confusion
Second reporter function: First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: We had just been on the aircraft a few minutes for the first flight of the day. We started the APU due to cold aircraft and no ground air. As soon as the APU was running; I got the disconnect signal from Ground Personnel. Parking brake was set and APU generator was online so I gave the disconnect signal. First Officer (FO) and I continued our cockpit prep as the Passengers began to board. A few minutes later we heard what sounded like a rattling of the tow bar against the nose gear. It was the normal sound you hear when they connect the tow bar; except it was a little louder and longer in duration. The FO and I commented to one another jokingly that we hoped they didn't rip the gear off as they hooked up the tow bar. Seconds later; there was a commotion in the back near the boarding door and someone was yelling STOP!  The A Flight Attendant stated that the jetway had moved on its own. I looked out the left window and the jetway with Passengers was right out my window instead of by the boarding door. I jumped out of my seat; stopped the remaining Passengers and asked them to step back while we sorted it out. I went up and informed the Gate Agent and on the walk down to the aircraft; he mentioned that the same thing had happened when they towed the aircraft to the gate earlier in the morning. As he repositioned the jetway back to the door; I walked down to inspect for damage.  When I spoke to the Ramp Agent; it was then that I realized it was the aircraft that had moved; not the jetway. He explained how the aircraft had rolled back over the chain that connects the set of chocks and had pulled the tug with it. I returned to the cockpit to investigate further. First I confirmed with the FO that the parking brake was set; which it was; and that it had been set the entire time; which he confirmed it had. The parking brake was set; the light was on; but the brake pressure only read 1;000 pounds.  My next call was to Maintenance; and through them we had a local Mechanic within a few minutes. His checks led to the discovery that we had a faulty Parking Brake Valve which allowed the brake pressure to bleed off very quickly. This was the end of it as we deplaned and left the aircraft for the Mechanics as it was not dispatchable without the parking brake; but there are some serious safety concerns I have about what transpired. We were extremely lucky the aircraft only rolled a few feet. It could have damaged the aircraft or worse pinched someone between the jetway and the aircraft door; rolled over something or someone; or rolled into something. There is no warning on the airport pages to have the parking brake set; even though we did have it set. Upon speaking more with the Gate Agent; he explained better what had happened earlier. A Supervisor was at the controls when they towed it in. They stopped and set the parking brake. It was observed by the folks around the aircraft that it moved. The person at the controls verified the parking brake was set and everyone just sort of dismissed the situation as though he may have just not had the brake set right away. I totally understand their point after seeing the aircraft roll with the brake set myself; it just didn't seem like it could have done it; especially with the tug attached.


Second narrative: [Report narrative contained no additional information].


Synopsis: The crew of a Boeing 737 reported that the aircraft rolled back while parked the gate with the parking brake set. A maintenance inspection revealed a faulty parking brake valve which resulted in the flight being cancelled.