Brakes 97 Sierra

Need help figuring out what's the problem.
97 Sierra 144000 mile, has been great truck.
About 4000 miles ago started to lose brake pedal and felt soft.
Put in remanned master cylinder. Felt OK for a few thousand miles then started to get the same problem again. Thought I must have gotten a bad rebuilt master cylinder. Replaced it again. it was worse, almost no pedal. After each master cylinder swap the brakes were throughly bled. Took it to garage. They said they checked everything and it had to be master cylinder. They put another rebuilt on and I have pedal but not as solid as it should be. If I stop and push down hard I can make it go almost to the floor. It definitely didn't do that before these problems started. I definitely don't want to pull my boat and trailer the way it feels now.
I have finally concluded that it has to be something other than the master cylinder. Nobody gets that many bad one in a row.
Help!!! Suggestions???? Thanks

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I had the same thing happen to my 97 gmc k1500. ended up being a bad wheel cylinder on the rear. replaced it disconnected the battery to bypass the anti-lock system bled it and its worked fine for the past 20,000 miles.

freddie is most likely right in the area, sounds like a leak. Easiest way to know is check you fluid level, if it is low, fill it, sit in the driveway with truck running, step on petal and hard but slow, see if it slowly goes to the floor, if it does, do this a few times and the get ut and look on the ground and inside of the tires, look for a oily or wet looking spot. This should show you the leak.