2005 2500HD Brake Problem

I replaced the front brakes a few months ago and the rear brakes (rotors) are original. Every time I apply brakes to stop the truck everything is fine until the truck is almost at a complete stop then
the brakes give in (depress a little further). I have replaced many brakes in my time and never had this problem. Can anybody give me some idea as to what is happening. Also, I don't know if this matters but I only put about 5,000 miles a year on this truck so I am wondering if the rear rotors may be warped and it's just hitting the warped point of the rotor when I am almost at a complete stop, although I never had the spongy sensation before I replaced the front brakes. Thanks
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Problems relating to "almost a complete stop" are usually and abs false activation. But your not mentioning abs... Rotor problems pretty much are always worse under heavy braking.
Almost stopped... that's when we normally reduce braking to almost nothing, for a smooth stop (and why it takes such a little bump from behind to roll you into an accident). I'm wondering if your assist is doing something at that point? Rise in vacuum or pump pressure, depending on what system you have?
My next thought... master cylinder leaking by? Can you duplicate the symptom when not rolling? Just step on the brake like you are stopping the vehicle, and let up as normally would as coming to a stop... see if it then sinks? If so, I'd be thinking master cylinder. If not, then the difference may be back to the assist.
Or, final thought... if you have a wheel bearing with excessive play, leaving the rotor running at a tilt. Then, brake pedal will drop as it stands the rotor back up. But, I think this problem normally is associated with immediate brake drop, not "as coming to a stop".
Hope this helps! PS... you didn't just change your shoe inserts to gel did you? : )

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