The normal brakes use the B system hydraulics and all fluid is routed from and to that system.
If you select Alternate Brakes then you are using A system and all fluid is routed from and to that system.
Some of the previous answers confuse the Power Transfer System with fluid transfer. The PTU uses A system pressure to drive a hydraulic motor which drives a pump in B system to increase the flow in times of increased need in the B System. No fluid transfers from A to B in that system.
If you apply the brakes with only B system hydraulics on, turn off B and turn on A then release the brakes the fluid will flow to the A return. Same for the reversers and spoilers.
There is an electronic shuttle valve to keep the systems isolated in normal operations but you can transfer fluid if doing it abnormally.
Here's a website that'll explain it to your heart's content
http://www.b737.org.uk/hydraulics.htm