Why do we have to do an alternate brake check during preliminary cockpit preparation?
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