B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 210; Flight Crew Total 10756; Flight Crew Type 4922
Communication Breakdown; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action; General Release Refused / Aircraft Not Accepted
Manuals; Procedure; Aircraft
On two aircraft; in two nights; I have encountered aircraft that have excessive brake pressure decay with the brakes released and hydraulic power off. In both cases the aircraft had to be taken out of service due to the decay because it was well out of maintenance tech order limits. Our flight manual says nothing about this and gives us nothing to look for to find this malfunction and we are forced to use a 'this doesn't look right' approach to writing up the problem. To me; not having guidance similar to the maintenance manual when encountering very low brake pressure indications during preflight is a major problem on a fleet that requires optimum braking performance on most landings. The mechanics in both cases gave us very tight limits for the decay which is allowed to happen in this case; and the problems that large decay might indicate are both leaks and improper valve positioning; both of which may have significant impacts on landing and braking performance.
The reporter stated on preflight he noticed that the brake pressure was low and the aircraft was sitting a very short time. After turning off the electric hydraulic pumps the brake pressure would bleed off very rapidly which was not normal. This was observed on two different aircraft on two different days. The reporter also stated that there was nothing in the Flight Manual that addressed this situation and felt that there should be. The reporter stated that maintenance control told him that this happened on another B737NG aircraft.
B737 First Officer reported two aircraft in two nights were taken out of service due to excessive hydraulic brake pressure decay.