I just went through a huge hassle because after replacing a line between the ABS unit and the brake hose I got a bit of air into the Kelsey-Hayes ABS unit on my 1994 G20.

You have to cycle the solenoids to get all of the air out. Parts of the system do not flow during normal use, but if they get air you will have a very low pedal.

You can mess around with a pair of special tools and bleed the unit manually (working blind under the van as bleeders are on top of the unit), you can track down an early diagnostic tool to try to do it - I had no luck accessing with an OTC 4000E even with an ABS cartridge in it - but there is a much, much easier way.

The same paperclip trick as one uses to pull codes, except you connect terminal A to H, which is bottom and to the left. It will be the only one towards the left side of the connector with a terminal in it. Connecting them will make the unit cycle just a bit. If you leave it connected, the ABS light will flash any codes in the system. Just like with the ECU a 12 is normal. Cycle it 4 or 5 times and that should be enough. It will have a good pedal again compared to previously.

Once you cycle it, bleed it again and it should be good to go. If you're using the Harbor Freight vacuum bleeder or a similar device, have a helper make sure the small reservoir (rear wheels) stays full, it doesn't hold a lot and you don't want to accidentally empty it and have to bench bleed the master. Mostly because 90% of the time that's probably going to mean replacing the lines from the master to the combo valve.

I struggled with this through the OTC and a MAC 2500 "brick" (which has some other problem as it won't connect and stay powered on) only to find out from Grok on the X app this was possible. Why it did not come up in Google or other searches I have no idea.

This is only needed if you have the 4WAL ABS unit. The 2-wheel ABS you can gravity bleed or use a HF vacuum bleeder on pretty easily.