Why do we have to do an alternate brake check during preliminary cockpit preparation? As part of the preliminary cockpit preparation in my company’s older A320s we have to perform an alternate brake system check but not in the newer ones as per FCOM Normal procedures as below. I found out that there is a system difference but didn’t fully understand could someone please explain the difference?


  ALTERNATE BRAKING SYSTEM
  Note: The purpose of this check is to
  verify, before the first flight of the day, the efficiency of the
  alternate braking system (absence of “spongy pedals”).
  Y ELEC PUMP
  pb-sw....................................................................................................CHECK
  OFF
  CHOCKS..............................................................................................................CHECK
  IN PLACE PARK BRK
  handle.......................................................................................................................OFF
  BRAKE
  PEDALS.....................................................................................................................PRESS
  Apply maximum pressure on both pedals.
  BRAKE PRESSURE (on BRAKE press
  indicator).................................................................CHECK
  Pressure must build up without delay symmetrically on left and right
  sides for the same application simultaneously applied on left and
  right pedals. With full pedal deflection, the pressure must be between
  2 000 and 2 700 PSI.
  BRAKE PEDALS................................................................................................................RELEASE
  PARK BRK
  handle........................................................................................................................ON
  The parking brake must be on during the exterior inspection to allow
  the flight crew to check brake wear indicators.
On the old system the switching between the normal braking using the green circuit  to the alternate braking using the yellow circuit is done by an automatic switching valve pressure dependant (Automatic selector).The alternate system being very little used, it happened the valve did not switch from green to yellow when loosing the green circuit

please refer to the old system here below

http://www.a320dp.com/A320_DP/landing-gear/images/braking-schematic-d.jpg

On the new system the alternate brake is activated by a specific computer(ABCU) decision and is automatically tested by the computer at power up as soon as the hydraulic pressures are available, please refer here below to the new circuit

https://m.facebook.com/A320Theory/posts/normal-brakingnormal-braking-is-obtained-when-the-green-hydraulic-pressure-is-av/790898894383508/

Hope this has been useful and explains why it was very important to do the testing on the old system