I have no doubt about your qualifications, and am not trying to be a smart@$$. I am going by what I have read IN THIS FORUM as being actual instances (note the use of the plural) of ballooning brake lines causing a mushy pedal in F150's.

 

I asked a bud of mine that owns a chain of brake garages and he looked at me like I was country stupid. He never heard of anything like that and he's been in business for over 30 years. Not saying it ain't possible with crap after market parts but it happenin' with factory or quality parts. Like teach says, there can be all kinds of other issues including that the tech didn't adjust the rod correctly and it has excessive over travel which allows fluid to go around the cylinder in the master cylinder. There is the issue of rebuilt crap parts and even some new units have leak down. Considering we have no clue what the guy put on the truck, what he did to it and neither does the poster, it's anybody's guess.

 

If you are certain you bled the air out the brakes and have eliminated MC by either replacing and/or bench bleeding and all lines are good w/o leaks and there is no ABS light on i'd highly suspect this.

I've been chasing this for over a year and finally have evidence that it's 99% the ABS HCU (the actual pump itself not the ECM on top of it)

This great vid explains what happens and you may not see any visable fluid leaks etc as it's all the internal fluid being by-passed

Fortunately i finally saw fluid leaking from the driver's side rubber plug under the hardlines so mine must be bad now!

A year or so ago i had the ABS light come on and the ABS HCU ECM was broken so replaced that and now again have the sinking pedal feel so i dug more into this and i'm sure it's the culprit!