B737 first officer reported two aircraft in two nights were taken out of service due to excessive hy...

Narrative:

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.

Original NASA ASRS Text

Title: B737 First Officer reported two aircraft in two nights were taken out of service due to excessive hydraulic brake pressure decay.

Narrative: 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.

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.

Attributes
ACN	1349790
Time
Date	201604
Local Time Of Day	1201-1800
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Parked
Flight Plan	IFR
Component
Aircraft Component	Normal Brake System
Person 1
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 210 Flight Crew Total 10756 Flight Crew Type 4922
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Published Material / Policy

B737 first officer reported two aircraft in two nights were taken out of service due to excessive hydraulic brake pressure decay.