One of my sons has a 1991 International 4700 non-CDL truck in his shop. It has no air brakes, and neither he or the boss have seen this kind of brake setup before. He just replaced the front disks and calipers, which are conventional, and the brakes work fine, but he's only test driven it unladen. When you step on the brake pedal, you hear a pump running on the firewall. After a few minutes test driving, a warning light on the dash says " Brake Press " The light stays on thereafter. Is this one of those dreaded Lucas Girling brake systems as found in school buses and Class 6 trucks? I was warned about them before in this forum.
Sure sounds like it based on your description. They aren’t all that bad. We’ve got one truck at work with them and I’ve got a Ford at my shop with them. Based on my experience if that pump is running and the light is on you have a couple potential issues….maybe have been run low on fluid and need bled or it’s got a leak somewhere and pressure is bleeding off. There are two wheel cylinders per side in the back and a hard line that ties them together, that’s often a good spot for a leak. We’ve been getting our parts for them from O’reillys believe it or not.