First off I wanna say first time poster long time fan!

I have a 97 F150 4X4 SuperCab Lariet w/ 4.6l and auto trans.

I have recently developed a mechanical problem that has me stumped.

I have a loss of brake pedal but the truck still stops. If I pump the pedal it gains pressure. When I am coming to a stop I get a loud clunk and it feels as if the rear end locks up for a split second. This usually occurs at the same time as my downshift, which at the time of my downshift and this clunk happens I get my pedal back. I am not loosing any brake fluid and have no leaks anywhere. This first happened while slowly backing up in reverse. KLUNK! I though i hit something got out nothing hit, drove for a couple months didnt happen again. Then out of the blue it did it again in reverse. Drove for about a month and one more time it did it in reverse and then it started doing it in drive. But only when braking sometimes its one klunk sometimes its a bunch in a row. I cant figure this out please help

 

your rear brakes are shot. Have them fixed.

 

Right. The 1/2 ton brake systems, drum and 4wheel disc work fairly well, but you HAVE TO keep up on them. There comes a time when you have to go thru the system and update all that original hardware, specially the 3 rubber brake lines as they break down inside then blow up like a balloon vs stopping the vehicle. You have a definite problem at the rear. Also a ABS problem that may be contributing. Clean brake fluid can also be just as critical.

Boils down to updating the entire system at a certain point which really isn't that expensive considering. Going thru the system thoroughly WILL definitely get you that worry free longevity. The parts within these systems have a major dependency upon one another. Meaning, the system will never work right again unless it gets the attention it needs. After 100,000 miles or so, it really needs to be gone thru. You'll be glad you did as you may be amazed what true stopping power really is and how much it can be a life saver.

For now, pull the rear wheels and inspect. With that clunking going on, I bet you'll see the problem(s) after a good look see...

 

loss of pedal could also be a failing master cylinder

 

Or deteriorated brake lines expanding well beyond normal.
New brake lines on my Exped made a huge difference in my "soft pedal" and felt a lot more responsive.