One thing that hasnt been mentioned (and please correct me if I'm wrong) is that the brakes hydraulics are controlled by the Sys B electric pump. That is why you turn that pump on right before pushback.

At American Airlines we turn our fuel and hydraulic pumps on just before push back. My Southwest buddy says they turn their A pumps off for push back. Oh yeah and I believe when we were talking about it my brother who works for Continental say they also leave their pumps on for push back.

One thing that hasnt been mentioned (and please correct me if I'm wrong) is that the brakes hydraulics are controlled by the Sys B electric pump. That is why you turn that pump on right before pushback.

Brakes are controlled by SYS B normally, sys A is an alternate source of pressure if B is not avaiable. (brakes are also a source for transferring A fluid to the B system reservoir).Please, talking about hyd system users, lik the brakes, don't use "elec pump" as the elec and engine pumps pressurize the same system. So Also the ENG 2 pump will power the brakes if it's running.The reason to have it on is for brake reason but is also because pilots turn on the pumps for turnaround and will leave them on if it is not required to unpower them. This will let also the hydraulic fluid to warm in cold days.