I have attempted to read the entire site of hints and suggestions on what to do with this issue. Hoping for some options.
2004 Heritage, 6 cylinder, no cruise, 110K miles
Brake pedal started to be kind of mushy and was sinking at a stop sign. No recent brake work, pads all okay, no visible signs of any leakage of fluid anywhere. No loss of fluid in brake master cylinder. All signs that a problematic brake master cylinder was the cause. So I replaced brake master cylinder! I was extremely careful with the bench bleed to remove all air. Manually bled all four wheels until all bubble/fresh brake fluid was coming out.
Still had the same problem. Mushy with the brake pedal slowing sinking at a stop. Got another new master cylinder, (both were a Duralast from Autozone) Checked the booster, no apparent problem. Bench bled the master cylinder again, manually bled the brakes again. Same feeling on the brakes after two brand new brake master cylinders. Checked all the lines, no visible areas of leakage or damage.
I'm at a loss, I can't imagine that I have two defective master cylinders. I had read somewhere in my travels that potentially an adjustment to the booster rod may help? Maybe a different vendor/maker of the master cylinder? Suggestions please!

 

I recommend that all of the flexible brake lines be replaced.

 

^^^

Yep. Get a helper to press and hold the pedal down while you crawl under and look at the hoses. I bet one or more will balloon.

 

its also possible that valves in your ABS module are going bad and you are leaking into your low pressure accumulators inside the ABS module.

I'd do the flex lines first (after 13+ years its a good bet to replace them anyway, plus they are cheaper)

If that doesn't fix it, then you might be needing a new ABS module.

 

Appreciate the point in a new direction! Thanks for the suggestions! I will check these out tonight! Thanks again!