On vehicles with rear drum brakes and front disc brakes the metering valve ensures that the drum brakes receive hydraulic pressure

The answer is "slightly before the disc brakes".

Explanation:

This dose appropriate secondary that perhaps the brake pads get hydrostatic fluid slightly even before brake discs on vehicles of rear trumpet brake and front disc brakes.

These same brakes are far more powerful and efficient than brake pads. It's why the brakes of disc-brakes could be found fading as much as on drum breaks.