EMB ERJ 145 ER/LR
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL
General Maintenance Action; General Flight Cancelled / Delayed
Aircraft; Human Factors; MEL
On Aircraft X from ZZZ our aircraft arrived with an MEL [for an] Accumulator Low Pressure Switch. When I got in the cockpit there was an EMERG BRK LO PRES (Emergency Brake Low Pressure) EICAS message that is associated with the MEL. The 'Brake On' light was also not illuminated; which is not associated with the MEL. I turned on Hyd 2 Elec Pump and the Brake On light illuminated. It went back out within 60 seconds of turning the Hyd 2 Elec Pump off. I called maintenance and told them I suspected the parking brake was not working. We were delayed for ATC flow by over 2 hours. I offered to push back and test the parking brake prior to boarding; which maintenance agreed with.While pushing back I turned hyd pump 2 to Auto to ensure the parking brake would work. I verified the Brake On light was illuminated before clearing the tug crew to disconnect. After we were disconnected we started engine 1. Then I turned off hyd pump 2 and waited to see if the Brake On light went out and the EMERG BRK LO PRES EICAS caution came on. The light went out and the caution message came on in less than 60 seconds. With hyd pump 2 turned off; engine 2 not running; the parking brake set on; and my feet ready to apply the pedal brakes; I slightly increased thrust on engine 1. We started to move right away. I stopped the aircraft immediately with the pedal brakes. I then turned hyd pump 2 to Auto and checked that the Brake On light came on. With the Hyd 2 Elec Pump running we returned to the gate. I called maintenance control back to tell them the parking brake was not working within 60 seconds of turning the Elec Hyd Pump off.The parking brake is supposed to last 24 hours or 6 applications on the accumulator alone; so lasting only 60 seconds shows there's a maintenance problem. I made a logbook write up and spoke to the gate agent; ZZZ operations; the on duty chief pilot; as well as our dispatcher. This is the third time I've faced this type of scenario. Each time I've reported it. The previous two times the problem ended up being a bad shuttle valve that allowed hydraulic pressure from system two to bleed off into system one. The danger of this scenario is we push back with hydraulic system two off. If engine two isn't started on the pushback; when the brake is set for the tug disconnect the hydraulic pressure will initially be enough to give a brake on indication. After 60 seconds; however; the hydraulic pressure will bleed into system one and the parking brake will not hold. If the aircraft were to roll during a tug disconnect a ramp worker could be seriously injured.The threat on Aircraft X was the parking brake was not working within 60 seconds of turning off the Elec Hyd Pump.Accumulator Low Pressure Switch needs to be modified to be absolutely sure the Brake On light is staying on with the hydraulic pumps turned off for a reasonable amount of time. Also; crews should be made aware of the potential danger of losing the parking brake if the Brake On light isn't illuminated when using this MEL. A note in the Operations Procedure of the MEL for the crew; and the Maintenance Procedure of the MEL for maintenance would help alert everyone to this potential problem.
EMB-145 flight crew reported parking brake inoperative.