proportional valve: how important to have working one?

[Frederick Smith]

I know my '88 5ktqa has one and so have all the others I have worked on.

The only one I had to mess with was while doing the rear brakes on my own car.
Since the car was on a single post frame lift and the rear axle was "hanging",
I had to raise/lower?? the lever on the spring assisted valve inorder to bleed
the brakes, since the axle position mimics the car in a nose dive stop.
Or..maybe I just removed the spring, but got no/little brake fluid from the
caliper 'till I did.