I will try and keep this short but I doubt I will be successful.

My 1999 Silverado developed an unusual (for me anyway) braking problem. At VERY low speeds of less than 5 or 6 MPH, the brake pedal will be almost impossible to depress but I can with a lot of foot PSI get the truck to stop. Lots of grinding sounds. Again, this problem never happens at speeds greater than 5-6 MPH. It will occur in forward or reverse. I know that the front brakes have never been serviced so I decided it was time. Upon inspection I noted that BOTH front rotors were abnormally worn or corroded on the INNER disc surface. There was a a smooth area in the "middle" ring of the rotor with this abnormal wear flanking it on both sides (talking disc perimeter rings here). The outside surface of the rotors looked fine showng normal wear. The pads of course were unusually worn. I removed the rotors, had them turned, and replaced the pads. The truck braked beautifully as I would expect.

Unfortunately, after a day or two the braking problem has again reared it's ugly head. That is, at slow speeds the brake pedal becomes rigid and I have to stand on it to make the vehicle stop. Recently at a toll plaza while creeping up in a line of cars it did NOT stop and I resorted to my emergency brake. Also, the braking system itself feels less than what it was after I serviced it. Spongy sometimes and nice and tight other times. It feels like a possible piston problem but I've never encountered these symptoms in any braking system.

its an ABS problem ware the speed sensors cannot raed the speed on the hubs properly due to rust and corrosion either pushing the sensor away from the hub or the sensor being dirty or both. There is a tsb on this and it is very common. the dealers will try to soak you for new hubs. I had mine done at the dealer and it was 300 or mor all labor. they will take the roters off clean the outside of the hubs and pull the sensors and clean them as well....JJ

Jake, you can check my other posts and see i know some things about brakes, not much else, but brakes definately, here is a general rule of thumb, if your rotors are grinding on the outside of the rotor and the inside has a considerable amount more pad left, then you probably have a hardware problem, if it is the inside pad that is grinding ( in your case) it is a caliper problem, sounds to me like you have 2 locked up calipers, what i would do is replace the calipers and lube all of you slides very well, and after you install the 2 new calipers, you will need to bleed the front brakes, now at this time you need to make sure that you are getting sfficient amount of brake fluid out of the bleeders, because it is rare to have both calipers lock at the same time, not impossible just rare, just saying this because you might want to make sure you dont have a collapsed brake hose causing these caliper problems...........here are some things to tell you , you have a locked up caliper,
1. piston wont push back into housing
2. pads grinding on inside and not outside
3. when you hit brakes it pulls to one side or the other

and if worse comes to worse, you can always take it to a place that does a free brake inspection, get their estimate with the parts needed on it, tell them you have to wait till you get paid, and take it home and do what they said it needed your self

As i understood your original post you did a brake job. The hard braking at low speed is exactly what happend to me and others here on the forum. Your sensors, not being able to sense the right wheel speed, activates the abs pump on these trucks and as you said you cant get it to stop. These events occure with out an abs light coming on nor a code being set in the computer. Just for FYI if you quikly pump the brake pedal once durijng the event it usually will stop the pump and it will operate normally. As i said this is a known problem that GM will not recall and fix(big suprise) and has been experienced by hundreds of owners who did not have caliper probs. Once the cleanig procedure was done at the dealer i had no more probs and this occured at 37000 miles. Like i said do a thread serch here in this forum and you will find the info and the Tech Service Bullitin from GM explaining it..JJ

PS the grinding sound you here is what the abs pump sounds like when it activates. Especiaaly if you fixed the pads and disks. Test this by jumping on the brakes in aparking lot at 25 mph or so, purposly activating the abs. The sound you here as the vehicle comes to a stop under the abs should be similer to what youve been hering at 5 mph..JJ

Just yank the fuse for the ABS and see if the problem persists at 5-6 mph. That's a sure way of knowing if its the ABS or not.

Mine's not grinding, but the inner surface of both front rotors are unusually worn. Was kind of thinking that the calipers are locking up. Unusual for both to do this at the same time but that's what appears to be happening. Going to do a brake job tomorrow.

More than likely it is the wheel speed senosr droping out at low speeds.
Gm has had a problem with this for awhile now. As someone said "It's rust build up on the hub pushing the sensor up" This creates an out of spec. air gap. You can remove the sensor form the hubs and clean the mateing surface. Be carefull the sensors will break off in the hub very easy. In some cases this will not fix it and you have a hub or abs problem. Yes, I too know alot about the brakeing system.