Help, brakes dragging, out of ideas

This is my friend's 97 1500 4x4 with a 5.9. Mileage is actually pretty low; around 150k. This problem has us stumped and I'm starting to obsess about it...

My buddy's truck developed a problem where the brake lights would stay on all the time. He uses it so rarely and short local trips so he never thought about fixing it till a few weeks ago.
We thought it was a bad brake switch... but no.
Come to find out the brakes were 'soft' and the brake pedal seemed to be riding too low. I assumed worn out brakes.
So we did a brake job all the way around... New rotors, calipers, and hoses on the fronts. New cylinders, shoes, drums and hardware on the rears.
Bled the brakes, adjusted the rears and tested.
Didn't help.
The brakes were better for sure so now when the pedal rode low the brakes actually drag. If you took your foot off the gas it would now stop the truck eventually. If you lifted the pedal with your toe the pedal would stay put and the brakes don't drag but as soon as you brake again the pedal will not come all the way up on its own.
Bled them a couple of more times with no change.
We then changed out the master cylinder and the brake booster. There seemed to be a little fluid between them, like slightly damp. And of course some grime. But if there was a leak it wasn't much of a leak.
It didn't cure the problem.
We've bled the ABS at least 15 times thinking that we got a little air in when we connected the lines to the master cylinder.
No change, pedal still wants to stay down a little and the brakes drag.
I pinched off the vacuum line to the booster with some vise grips and now the pedal comes up.
So too much vacuum? I've not measured the vacuum with a gauge but it 'feels' about right for manifold vacuum.
But since pinching the line doesn't actually reduce vacuum... more like volume it seems to me, eventually, if you drive it very long, it will start to drag again.
I keep thinking we still have air in the abs, rwal valve or the combination valve because when idling, for instance at a stop sign, if I pump the brakes 3-4 times the pedal comes up and stays up.
But I've bled them a bunch of times and when the trucks not running the brakes are hard and the pedal stays up.
Like I said I'm outa ideas.

Did you change out the master cylinder and booster with new or used parts?

Also not sure what configuration the return spring in your application, but maybe the old one was gone and the new parts didn't come with one, (expected you to install the old one, or it isn't part the master cylinder/booster assembly and resides elsewhere)....

New.
Don't think there's a separate return spring. Both the master cylinder and booster are sealed assemblies.
But I will do a search for a 'return spring' part and see if there's something like that out there.
Thanks.

So I did a search for a 'brake pedal return spring' and I'm not finding it at any of the factory parts sites... after my exhaustive search of 2 sites that took like 3 minutes.
Truth be told, we thought about adding one awhile ago. But before we re'engineered the brakes we decided to try and fix it.
However, my search for a spring gave me a lot of links to forums with posts about 'slow brake pedal return' and 'brake pedal sticking'. But so far nobody's posted how they fixed it.

you need to figure out if the problem is front brakes dragging or rear brakes dragging. Jack up each end and spin each wheel to feel where the drag is.

I would be curious to see what happened if you disconnected the brake booster hose. The symptoms you describe are similar to what some chevys do when the seals start to give out. It takes a minute and will quickly eliminate a large portion of the braking system from the equation.

If that isn't the case, there are plenty of other suggestions here to try.

dudeman2009 said:

I would be curious to see what happened if you disconnected the brake booster hose. The symptoms you describe are similar to what some chevys do when the seals start to give out. It takes a minute and will quickly eliminate a large portion of the braking system from the equation.

If that isn't the case, there are plenty of other suggestions here to try.

Click to expand...

This was my first idea that i halfway discarded and why I asked if the parts were replaced with new.

Replace the rear brake hose.

Thanks for the replies.
If I disco the vacuum line from the booster the pedal comes up all the way and the wheels turn freely. This is essentially the same as pumping the brakes and checking them with the truck not running.
And, as one would expect without the booster, if you're driving you have to use both feet pushing on that pedal to get the truck to stop at pretty slow speeds (10-15 mph max).
Which wheels drag? All 4. The pedal is sucked down an inch or so from the switch so it's equivalent to someone driving with their foot resting on the brake pedal. If the 4 wd is engaged and it's up on jack stands, in drive, and you push the pedal down, it stops all 4 wheels. Normal. If I release the pedal they stay stopped. Not normal.
If, instead of keeping my foot on the brake till the wheels quit turning, I give the pedal one quick hard push, the pedal stays down enough that it will eventually stop the wheels from turning.
If I shut off vacuum none of them drag. If I shut off the truck none of brakes drag.
So I think I'm missing something on the one suggestion... If I disco the booster I'm just eliminating the vacuum assist, I'm thinking. All the hydraulics and sensor feedback is still operating. Poorly, mind you, but they're still part of the system.
I did forget to mention it's not throwing any ABS codes.
Oh, lol, there is no rear brake hose. Hard line all the way from the wheel cylinders to the MC. Front hoses were replaced at the beginning of this little project.
Something bugging me is I had the same thing happen on an early 90's Ford pickup years ago. Pretty much the same system - rear wheel anti lock, drums on the rear and disk brakes on the front. I replaced the oozing master cylinder and booster, did a brake job and that was that.
And this really bugs me:
Drive for a block or so to a stop sign. If I pump the brakes while I'm at a stop with the engine running, within three strokes I've got full hard pedal and when I take my foot off and hit the gas the brake pedal is all the way up, stays up and no brakes are dragging. To me this means air in the system but my buddy thinks I'm nuts because we've bled the wheels and abs so many times.
I have also found, when searching forums, that most folks are calling this problem 'sticky brakes' when they post a similar issue. So far it seems that people corrected with what I've already done or in the cases where it was still acting up they didn't post what finally fixed it. Yet.
If someone sees something different post a link? Please?

first, there has to be a flexible line between frame and axle housing, because the axle moves up and down relative to frame.
second, I think you have failed to remove all the air from the system. Either you are bleeding the brakes wrong or there is a leak in the hydraulic system. Incidentally, I read that you are supposed to put a block of wood under the brake pedal to prevent over travelling the m/c piston and damaging the seals.

First thing I would do is buy some brake line pinch pliers. Vise grips will work in a jam. Next pinch all three rubber lines off. One at each from wheel then the one that runs from the frame to the rear differential. With the lines pinched test the brake pedal. This has isolated the master cylinder, booster, abs. If the pedal is firm un pinch one line at a time until the soft pedal returns. If the pedal is soft with all of the lines pinched the problem is before the wheels. Un pinch each line one at a time testing in between until the pedal does its low sticky thing. Bingo then you at least know where to concentrate your efforts.
I know I've seen some cars actually suck in a little air at the abs system, Honda is notorious for this. What your describing however sounds like a brake booster or misadjusted brake rod, between the booster/master cylinder, or from the brake pedal to the booster.