Rubber brake hoses(one at each caliper, one connecting the forward lines to the ones on the rear axle housing)
Wheel cylinders (will have to remove drums to check)
Brake calipers (removal likely required)
Master cylinder (could be leaking through the rear seal to the firewall)
Hydroboost unit (most van-based GM chassis had these, rarely have room for a vacuum booster, with a diesel, it is standard)
Depending on the chassis your bus was built on, you may have an oddball problem. GM trucks had a strange sort of front-only anti-lock system from at least 1990-1992. It was mostly touted on the S-trucks, but I believe this madness may have extended to the C/K, G and P trucks / vans as well, and may well have been done as late as 1994. They were primitive and flaky, and known to do some very weird and stupid things.
Years ago, I had a GM sedan (two, in fact) on which the brake systems suddenly developed a mind of their own. On one, jamming on the brakes gave a different result each time -- one time it would lock the fronts but not the rears. Next time it might lock the right side, but not the left. Another time it would lock opposite corners. VERY mushy pedal as well...
Never did find the problem, but I'm pretty sure it was a proportioning valve (interestingly enough, in the master cylinder, which had been replaced and bled. However, I understand GM had a little-known attempt at 'simulating' anti-lock brakes through some sort of hybrid proportioning valve. Perhaps the parts counter monkeys didn't know about this, or how to look it up.
The other, just simply had a weak pedal and about 1/3 the normal braking power. Systems were completely redone on both cars, but they never regained full braking power. I suspect this one had the same problem. Wound up scrapping both.
Look for an RPO sticker (option code build sheet -- G-vans had them inside the engine doghouse console lid) and find out which brake system yours was built with. If you can't find it, you might contact a dealer with the VIN to see if they can help you. Depending on how yours is set up, if it is such an issue that you can't find OEM parts for, you may be able to swap to an older system on the same body style.
One other thing, you might try running your VIN through this site, no guarantees it will work if it was an incomplete vehicle upfitted with a non-GM body.