Just got a 98 Suburban, 6.5TD, 2500. What a nice truck! My only issue so far is the brakes. I have had then fail several time so far, luckly I have not dinged anyone yet! It usually happens during low speed braking while going over a bump in the road. The ABS seems to kick in and release the brakes for a split second. What a feeling! You automatically stand on the brakes which causes the wheels to lock-up more and ABS to counter more. When I first got the truck the front tires were at 80PSI and the ride was real rough, I lower to 50PSI and this brake issue seemed better, but still happened occasionally. It just happened again last night, it was raining and I went to break at a stop sign and the truck took an extra 50 feet to stop. Thank God there was nobody in front of me! This truck is just not safe to drive.

Anybody else experience such failures? What can be causing the breaks to just let go?

I had a similar problem on my 99 Suburban. Abs would come on for no reason occasionally. No Abs light would come on either. Turned out to be a bad front wheel sensor. I diagnosed it by checking the resistance of the sensor as I rolled the wheel. One sensor was out of range. It worked well enough that it passed the sensor check when you start the truck (no Abs light) but would fail occasionally and trigger the Abs. Changed the sensor and no problems since.

Rust build up under the front wheel sp sensors

Maybe a pump volume or pressure issue?

Try removing the sensor and cleaning the sensor and the hub with pressurized air. If that doesnt work you can spend 60 bucks to replace the sensor(possible) or like 200-300 to replace the entire hub.
Or you can do it the easy way and pull the ABS fuse and remove the bulb that illuminates the ABS light and be fine.

You have a low speed abs issue
Bill