Extreme Flight Simulation - Chicago

The 737's landing gear runs on Hydraulic System A. (Easy way to remember it: the word "landing" has an A in it. Landing = System A.)

When the gear lever moves up, a specific sequence kicks off — and the order matters.

First, anti-skid deactivates. This has to happen before anything else, because the next step depends on it: the brakes momentarily clamp the wheels to stop them from spinning.

There are two reasons the wheels need to stop before retraction:
A high-speed spinning tire entering the wheel well is a hazard — if the tire is damaged, it can tear through nearby systems. And a fast-rotating wheel resists being repositioned, putting significant load on the gear struts.

Once the wheels are stopped, the landing gear valve opens and hydraulic pressure pulls everything up into the wheel well. Uplocks then engage, mechanically locking the gear in place. At that point, hydraulic pressure is no longer needed.

On the 737 NG, the pilot moves the gear lever to OFF to close the valve manually. On the 737 MAX, that step happens automatically — which is why the MAX gear lever only has two positions: UP and DOWN.