Narrative:

Took the aircraft from an inbound crew. My review of the logbook revealed an unusual number of hydraulic leak issues (five or so) with associated quantity servicing. Systems a and B were each implicated at least once. Hydraulics were off and APU was off at this point. Digital system pressure on both a and B read 10 at that point; which is unusual - usually so soon after a flight with pumps off; the systems would read about 60 psi - trapped head pressure in the reservoirs. I decided to investigate the gauge in the wheel well; not wanting to have hydraulic problems in international airspace. Analog hydraulic reservoir gauge pressure read zero - just what I suspected given the indications in the flight deck. I believe the aircraft had a leak on the pneumatic side that pressurizes the reservoirs. I called dispatch and conferenced maintenance who sent out a contractor.with hydraulic pumps off; we cranked the APU and opened its bleed line; and saw a response in the flight deck on the a and B system digital readouts - now showing 30 psi - about equal to the duct pressure from the APU. Under guidance of the mechanic; we turned number two engine and saw a further response on the digital pressure readout - jumping to 40 psi. According to the mechanic; he saw similar indications on the analog gauge. We then tested the system under a bleed load by turning on both packs. When we did this; we again saw a response in the digital readouts - from 40 to 20 psi on both a and B systems and the mechanic reported it went down to 10 psi on the analog. When we turned off the packs pressure jumped again on all three locations (my two digital in the cockpit and his analog). Shutting down the engine and turning off the APU bleed caused the gauges in the cockpit to return to 10 and the analog gauge in the wheel well to dissipate quickly to 0.it was decided by maintenance control that they would put the aircraft on an MEL for a bad pressure gauge. None of what I witnessed convinced me that the gauge in the wheel well was bad; it was reading exactly what I was seeing on the digital system readouts on the cockpit. When the contractor told me they were going to MEL it under MEL 29-xx 'hydraulic reservoir air pressure indicators (wheel well);' I did not agree with the determination nor the application of that MEL to this scenario. I called dispatch to prevent the re-planning [of] the flight because I disagreed with the MEL and refused to fly the aircraft.while waiting for my next aircraft to arrive; I called maintenance control to follow up and try to fully understand the MEL they applied and their rationale. They said they followed the book and troubleshooting steps (boeing maintenance manual) which brought them to the conclusion step that the gauge was bad and they could meled it. I asked him how he could still conclude that knowing that the cockpit indications corroborated what the analog gauge was saying; and he did not have an answer other than they followed the manual and this is where it lead them; directing them in the end to look for a leak in one of several lines which was not found. I asked him to weigh what I was telling him in addition to his manual and consider that maybe it was worth considering that the manual might be inaccurate and to take a fresh look at it--before they released the aircraft for another flight.I remain convinced that [this] aircraft is flying with an inaccurate MEL; that the hydraulic head pressure system that is leaking; and that it will continue to have hydraulic issues until the head pressure system is more critically and accurately diagnosed.

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Original NASA ASRS Text

Title: Captain reported a disagreement with Maintenance Control over a hydraulic system MEL and refused the aircraft.

Narrative: Took the aircraft from an inbound Crew. My review of the logbook revealed an unusual number of hydraulic leak issues (five or so) with associated quantity servicing. Systems A and B were each implicated at least once. Hydraulics were off and APU was off at this point. Digital system pressure on both A and B read 10 at that point; which is unusual - usually so soon after a flight with pumps off; the systems would read about 60 PSI - trapped head pressure in the reservoirs. I decided to investigate the gauge in the wheel well; not wanting to have hydraulic problems in international airspace. Analog hydraulic reservoir gauge pressure read zero - just what I suspected given the indications in the flight deck. I believe the aircraft had a leak on the pneumatic side that pressurizes the reservoirs. I called Dispatch and conferenced Maintenance who sent out a contractor.With hydraulic pumps off; we cranked the APU and opened its bleed line; and saw a response in the flight deck on the A and B system digital readouts - now showing 30 PSI - about equal to the duct pressure from the APU. Under guidance of the Mechanic; we turned number two engine and saw a further response on the digital pressure readout - jumping to 40 PSI. According to the Mechanic; he saw similar indications on the analog gauge. We then tested the system under a bleed load by turning on both packs. When we did this; we again saw a response in the digital readouts - from 40 to 20 PSI on both A and B systems and the Mechanic reported it went down to 10 PSI on the analog. When we turned off the packs pressure jumped again on all three locations (my two digital in the cockpit and his analog). Shutting down the engine and turning off the APU bleed caused the gauges in the cockpit to return to 10 and the analog gauge in the wheel well to dissipate quickly to 0.It was decided by Maintenance Control that they would put the aircraft on an MEL for a bad pressure gauge. None of what I witnessed convinced me that the gauge in the wheel well was bad; it was reading exactly what I was seeing on the digital system readouts on the cockpit. When the Contractor told me they were going to MEL it under MEL 29-XX 'Hydraulic Reservoir Air Pressure Indicators (Wheel Well);' I did not agree with the determination nor the application of that MEL to this scenario. I called Dispatch to prevent the re-planning [of] the flight because I disagreed with the MEL and refused to fly the aircraft.While waiting for my next aircraft to arrive; I called Maintenance Control to follow up and try to fully understand the MEL they applied and their rationale. They said they followed the book and troubleshooting steps (Boeing Maintenance Manual) which brought them to the conclusion step that the gauge was bad and they could MELed it. I asked him how he could still conclude that knowing that the cockpit indications corroborated what the analog gauge was saying; and he did not have an answer other than they followed the manual and this is where it lead them; directing them in the end to look for a leak in one of several lines which was not found. I asked him to weigh what I was telling him in addition to his manual and consider that maybe it was worth considering that the manual might be inaccurate and to take a fresh look at it--before they released the aircraft for another flight.I remain convinced that [this] aircraft is flying with an inaccurate MEL; that the hydraulic head pressure system that is leaking; and that it will continue to have hydraulic issues until the head pressure system is more critically and accurately diagnosed.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.