For the 300/400, dolly737's on the money and I got a thumbs up from an engineer.
Mr Boeing says:
Hydraulic system B pressure is available for raising the landing gear through the landing gear transfer valve. Hydraulic system B supplies the volume of hydraulic fluid required to raise the landing gear at the normal rate when all of the following conditions exist:
- airborne
- No.1 engine RPM drops below a limit value
- Landing Gear lever is positioned UP
- either main landing gear is not up and locked.