Awful hop

[Matt's'93]

Vehicle: '93 extended cab W350 DRW 5speed traction bars and flatbed.
Problem: So my pickup drives fine (drives straight, stops straight) but when I push the brake pedal all hell "brakes" loose. It bigins hopping, not really wobbling, but mostly an up and down feeling. It's bad enough I can see my hood/bed moving up and down. I have good Toyo M-55 tires I bought new that I have had balanced, rotated, and checked for out of round several times. I have also put on new front pads, calipers, rotors, and checked bearings. I have also replaced rear shoes, turned drums and bearings in rear. All brakes have been flushed bled and adjusted. The pedal is firm and consistent. The only drivetrain modification is a set of traction bars that had no effect on this issue.
So...... Any ideas? Again it only happens when I brake.

[BILTIT]

Check your axle ubolts and centering pin.

[Matt's'93]

Been there, all good to go...

[BILTIT]

Remove the traction bars and see if its still there, probably some kind of binding going on.

[Matt's'93]

If I remember right it was there before the I built the bars. But that is a good idea I will try that!

[RCCUMMINS89]

might be bushing related....

[GO OVRIT]

Are your springs curved evenly, or do they have a slight s shape? Like rccummins said, are your spring eye and shackle bushings tight? I wonder if there's any chance the rear abs could cause it.

[Matt's'93]

Well my front springs have a slight downward arch to them. All the bushings seem round with no slack in them. And this last weekend I disconnected power and the sensor to my RWAL and it still persists. I have noticed a clicking in the front end that goes away with the tiniest of brake pedal input.
Any thoughts about my master cylinder bypassing itself and fighting with pro portioning valve?

[RCCUMMINS89]

Sounds like bad brakes and warped rotors...............but you've replaced those, how are your steering knuckles (kingpins)?

[ellis93]

Just because they're new doesn't mean a thing,all it takes is one butthead to stand them up on the shelf instead of laying them down. BTDT. Use a dial indicator and measure the run out.

[GO OVRIT]

Were they new rotors, or did you have yours turned? Either way, they could still be out. Do you have any slack in the tie rod ends? If there is, the tires will fight themselves, but you would feel that in the steering wheel. Are you able to narrow it down to the front or the back at all?

[RCCUMMINS89]

True that...... I just replaced my tie rod ends and magically fixed my out of round tire problem.

[Matt's'93]

So when I did my brakes I purchased new rotors from the parts house, along with re-manufactured calipers and new pads. I believe it is in the front end. All of my tie rods seem to be tight and my ball joints don't have any side to side slack when I jack it up. I think that I am going to take the front brakes apart again to check to see if my calipers are sliding back when I let off the pedal. Previously I had one on the passenger side front dragging which in turn warped my rotor leading to the brake job.
Thanks for all the suggestions I will keep working at it.

[Matt's'93]

So I checked all my brake work and the passenger front wheel spins freely, and then hangs just a little. Would this be a warped rotor again? When I did my brakes originally this side would hang up so bad you couldn't turn it by hand so I am not sure if a little bit is ok? Also I just noticed that the bracket holding my steering box has a slight crack in it at one of the mounting bolts. Could this be the culprite to my issues...

[Napoleon's93]

I have a similar problem with mine. When I depress the brake pedal the brakes seems to catch and release on a very regular interval that is relative to my speed causing the truck to hop or lurch. I wouldn't describe it as a vibration at all. Mine happened rather suddenly as well, one day it was fine and the next it all went bad and has been ever since. There wasn't a gradual degradation of failure. I had my brakes completely redone less than two years ago so I am taking it back to the shop that did the work and we'll see what we find. I can report back here on our findings because your issue sounds identical to mine.