98 K2500 brake question

I posted this on the Sierra forum, but seems that there's a lot more traffic here. Please forgive the cross-post.

Hi,

I have a '98 Sierra 2500 HD (8 lug) 4x4. Today, the brakes went out. Seemed to be a bad master cylinder. No fluid leaks at the front calipers or the rear wheel cylinders, and plenty of fluid in the resivoir, but the pedal went right to the floor. Took it to my mechanic, and we both felt that it was a bad MC (looked to be original, truck has 145k miles). So, we get a new MC, and bench bleed it. Swap the new MC in for the old one, bleed it, but still no pedal. Won't pump up, and goes right to the floor. We bled all the lines, but still no pedal. My mechanic said that he flet very little pressue when bleeding the lines.

I was wondering if there was possibly something in the ABS unit, or some kind of vavle that could be bad.

Any thoughts/insight on this would be greatly appreciated.

TIA and happy holidays.

We looked at it some more later tonight. ALLDATA talked about some kind of valve in the ABS controller. Anyone know about this? It seems liike some kind of mechanical problem as the pedal will go to the floor w/ the truck off. It's got some resistence, but it will still go to the floor. It just goes faster w/ the truck running.