B737-700
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action; General Flight Cancelled / Delayed
Aircraft; Company Policy; Human Factors; Manuals
During my Captain preflight checks; I turned OFF the A System Hydraulic pumps and turned ON the B System Hydraulic pumps. I then checked the system pressure and it indicated 2780 psi on the B System side. That seemed low to me; so we referenced the B737NG AOM Preflight section. The AOM stated; 'Verify pressure and quantity;' but no minimum pressure. We then referenced the FRM (Fault Reporting Manual) B737-700/-800 and it indicated; 'Normal pressure - 3000 psi.' We decided to call MDW Maintenance to have someone come out to the aircraft to verify if this was considered 'normal' pressure. I wrote up the discrepancy in the Maintenance logbook as the 'B System Electric Hydraulic pump indicates only 2780 psi.' [Maintenance] came to the aircraft and could not find a minimum pressure in his manual; so; he called [Maintenance Control] to verify and get a reference for sign-off. After a few minutes [Maintenance] came back to the aircraft and told me [Maintenance Control] relayed to him the minimum hydraulic pressure for the Electric Hydraulic pumps is 2800-3000 psi. Our B System was still indicating between 2750-2780 psi. [Maintenance] then asked if I would be willing to change my logbook writeup to read 2800 psi. I declined. During the maintenance troubleshooting; they discovered the Brake Accumulator pressure was also in the red and indicated almost the exact same pressure. Ultimately; the [Maintenance Control] decided to take the aircraft out of service and we were given a new aircraft. I believe a minimum Hydraulic Pressure for the applicable AOM preflight section should be included. 'Normal pressure - 3000 psi' can be misleading and inadequate. Additionally; being asked by a maintenance technician to falsify an aircraft logbook write-up is a safety issue and a systemic problem here at [the company].
B737 Captain reported hydraulic system fault during preflight.