It has a level sensor on the rear axle that restricts flow when there isn't a load. I popped mine off (the rod that is) and lifted it up to bleed. When that wasn't enough (I had a broken line in the back that I replaced) I loosened the return line under the pressure differential valve (under the brake booster/master) and got LOTS of air from there. When I went to 33" tires (33/9.50/15) i removed the return line (plugged it with a cap on the PD valve by the master cylinder and bypassed the load level valve in the rear) The drawback to this is the level sensor is an anti-skid device so you have to be careful in the rain. With better front brakes (off a ZJ) and an upgraded master/booster ('96 XJ) I can stop at 55 mph while pulling a 3500 lb load while having a motor/trans in the back of the truck for "ballast".
Comanches didn't get factory ABS. Just the rear anti-skid valve.
"Usually found in a Jeep near Reiter, WA"
FOR SALE STUFF
'81 Eagle wagon with upgrades $2500
'87 Comanche shortbox 4.0/AW4/NP242 on 33" tires $3500