I would think if it where the hydroboost, i'd still have breaks but it would be like I had no power assist. This is not what I am experiencing. I am now 1500 miles into my 3 week vacation carrying my camper and towning an ATV trailer and here's what I know.

It happens all the time now.
It happens only when the vehicle's speed drops well below 10 mph (usually around 7 or 8 mph).
It starts as a distinct "clicking" coming from under the hood and the brake pedal hardness happens almost simultaneously.
I dont have to be pushing on the brake pedal for the clicking to start (at least I don't think so). When this happens, again usually around 7 or 8 mph (definitely below 10), if I THEN push on the brake pedal, it is already hard and pushing back.
If I have my foot on the pedal with constant pressure so as to be slowing the truck down through 10 mph, it feels as though the brakes actually "let go", that is, they are not longer actively slowing the truck, as though I simply let off the brakes.
The pedal then pushes back up against my foot and almost vibrates. If I continue to keep up the pedal pressure I can eventually push through it and the truck will then stop abruptly (imagine rolling along around 5mph and then hitting the brake pedal abruptly, like that.)
The reapplication of the brakes conincides with the stopping of the clicking noise that doesn't just "cut off" but more like fades away.

 

Check the fluid in your power steering pump first.
I had the same problem that you are having, only my pump was also making noise. turned out to be a bad steering box. Dealer hung a new box and told me to go on my way, I argued with them that the pump had to also be bad and they said no. I have about 70,000 more miles on it now and still no problems from the pump.
Also take a look at the wire that is suppose to be clipped to your front brake lines as they go through the wheel opening. if one brake loose it can become damaged by the wheel and short out on you. typically this will throw an ABS light on the dash though.

 

A quick update. Today I took the ABS fuse out of the block. That of course immediately set my BRAKE and ABS dash lights ablaze. But it also seems to have solved my problem! I've put about 25 miles on the truck without so much as a twitter from the brake pedal! Truck stopped smoothly and quickly. No hard pedal, no nothing. No matter what I tried.

So????

 

Check my previous post in this thread. my truck had the exact same symptoms as yours did. Turned out the passenger side front wheel speed sensor was bad. It did not trip the ABS light or show any signs externally that it was bad.

Given that it was not turning on the ABS light I would question if scanning the ABS control module would show which sensor is bad. Assuming you are not having problems with your speedometer you can likely rule out the rear sensor. The two fronts are the same part number so if you bought one (~$65 list at the dealer) and were lucky enough to get the old ones out without breaking one you could swap it side to side and figure out which one is giving you trouble.

In my case I had previously changed the drivers side front when I did ball joints (it was damaged while removing the wheel bearings). I figured I would gamble the $65 on the sensor instead of on the dealership scan of the control module.

This morning I sat down to suggest you remove the ABS fuse and see how it goes. Looks like you beat me to it... Good deal that it stopped it from happening. Best of luck.

 

Wished I'd have pulled it out about 1000 miles ago before I headed home. Would have made the drive less hair raising (drove thru NJ, for those in the northeast, you know what I mean....)
I'll consult my Haynes on how to R&R the sensor but it's sounding more and more like you're right. And it's in keeping with what was my initial thinking.