Soft pedal after hard turns

[John S]

71 Duster. Did a SSBC 4 wheel disk brake conversion. Complete kit modeled on the Kelsey-Hayes calipers. 15" Rallye wheels. LBP.
In general, the car stops great & straight.
If I jerk the steering wheel, take an exit ramp aggressively, or hang a hard left or right, the first pedal push is soft and goes about 1/2 to the floor. After that, it is firm to the top and as long as I drive in a straight line, it remains hard.
So that means the rotor is flexing on the turns and pushing the pistons back in.
We have gone back through and checked for loose bolts, play ,wheel bearing tightness. The only thing we found was the wheel bearings were a little loose and one control arm was slightly bent. Fixed and was a smidge better but not much.
We've called SSBC and they are stumped.
The 30+ year mechanic I'm working with says this was a common issue on old Fords that used the same KH system.
I know something is flexing but am stumped as what to try next. Or is this just an inherit issue that is in these systems?
Any body else experienced this or have suggestions?
Thanks, John

 

[Dana67Dart]

Disks rely on the flex of the axle / spindle to move the piston away from the disk a few thou.

Perhaps you have too much play in the spindles etc and the piston is getting pushed too far.

Also the optimum angle is 45 deg for the caliper.

Horizontal doesn't see much flexing, vertical sees too much.

This info was in a race suspension book from the 80s I had

 

[John S]

Yes. I'm pretty sure that is what's happening but don't know what is flexing... Or if this was normal on these old style systems.

 

[1WildRT]

Yes. I'm pretty sure that is what's happening but don't know what is flexing... Or if this was normal on these old style systems.

Normal? No... Check the wheel bearings for any movement on all 4 corners....

 

[jos51700]

Rotor shouldn't flex on turns, no force goes through it outside the hat.

Lug nuts tight?

 

[John S]

Normal? No... Check the wheel bearings for any movement on all 4 corners....

Yep and there was. Tightened and it helped a little.

 

[AJ/FormS]

Did you install the bearing races? Are you sure they are all bottomed in the bores?

I usually run my bearings just a hair loose, for a wee bit of knock-back. This helps in the fuel-economy runs. It doesn't seem to hurt the bearings cuz mine have over 130,000 miles on them.
But, my pedal doesn't sink as far as yours.

 

[John S]

Rotor shouldn't flex on turns, no force goes through it outside the hat.

Lug nuts tight?

Yes. I even use a torque wrench. We've pulled the wheels prob 4 times now checking and testing.

 

[John S]

Did you install the bearing races? Are you sure they are all bottomed in the bores?

I usually run my bearings just a hair loose, for a wee bit of knock-back. This helps in the fuel-economy runs. It doesn't seem to hurt the bearings cuz mine have over 130,000 miles on them.
But, my pedal doesn't sink as far as yours.

It originally went 1/2 way but when we tightened the wheel bearings, it did get a little better. The harder I turn the wheel or hang an exit ramp, the more pronounced the pedal drops is. Do you have any extra slop after a hard turn? Using KH style setup?

 

[1WildRT]

It originally went 1/2 way but when we tightened the wheel bearings, it did get a little better. The harder I turn the wheel or hang an exit ramp, the more pronounced the pedal drops is. Do you have any extra slop after a hard turn? Using KH style setup?

What AJ is suggesting (and it's a good point) if the cup weren't driven fully hard against it's seat it could shift slightly allowing additional bearing play...

Might be a good ideal to pull the rotors & check if the cups are fully hard against their seats..

 

[AJ/FormS]

The harder I turn the wheel or hang an exit ramp, the more pronounced the pedal drops is. Do you have any extra slop after a hard turn?

I do not run them THAT loose, lol.
IIRC the spec is .001 to .003 end play. It's just enough that it closes in on Zero when the bearings get warm.
I think the last time I adjusted them I set them to not more than .007, it depends on if I can get the locking-parts to line up with a cotter-pin hole.
And I use top-quality grease, no gun-grease here.

If yur bearings can't hold the adjustment, something is wrong, attend to it ASAP.

EDIT
I checked and the endplay is actually ZERO to .003.
The procedure is to tighten to 90 inchpounds while rotating the disc; then back off by one slot, and slip the cotter pin in..

 

[John S]

I do not run them THAT loose, lol.
IIRC the spec is .001 to .003 end play. It's just enough that it closes in on Zero when the bearings get warm.
I think the last time I adjusted them I set them to not more than .007, it depends on if I can get the locking-parts to line up with a cotter-pin hole.
And I use top-quality grease, no gun-grease here.

If yur bearings can't hold the adjustment, something is wrong, attend to it ASAP.

I think you are right. It almost has to be this. The mechanic that mentioned old Fords w the same style had the same issue... has this been a design flaw from way back when that is still occuring because 1) old stock parts and 2) copy of the KH calipers? Or is my play more than what factory cars had back then? Yes, we had the same issue with the cotter pin alignment being in the wrong place. We did use that clear high temp grease that is used on circle track racing. I was also wondering if I have a set of wheel bearing that are just made poorly. They are new but that doesn't mean anything these days. Thanks!

 

[Dana67Dart]

Rotor shouldn't flex on turns, no force goes through it outside the hat.

Rotors not flexing spindle is or wheel bearing play

 

[John S]

Rotors not flexing spindle is or wheel bearing play

I think I'm going to get another set of wheel bearings to rule them out.

 

[Dana67Dart]

Power brakes or manual?

Power maybe your vacume is lower when you Come out of a curve?

1 way valve on booster has a leak and not holding vacume.

Under idle conditions lots of manifold vacume

Under heavy acceleration vacuum drops. Leak in 1 way valve drops vacuum in booster, drop off gas heavy on brake not enough time to recharge the booster

 

[AJ/FormS]

IMO, first reset the endplay to near zero or even a tiny bit of preload, secure the locking system, then roadtest it.
>If/when the problem returns, check the endplay. If it has changed, then unless the bearings are soft a butter, most likely the cups are still not seated
I use the old cups to press in the new. If you don't have a press, I have a machined block that I drop in there and pound the back to back cups in. On my bench, When the cups bottom, the hammer bounce-back changes, and then I know it's in.
Don't forget the other side.
>If the endplay DOESN'T change, and the pedal-problem remains; then IDK what is going on. But remember to put the backlash back.

EDIT
I checked and the endplay is actually ZERO to .003.
The procedure is to tighten to 90 inchpounds while rotating the disc; then back off by one slot, and slip the cotter pin in..

 

[John S]

Power brakes or manual?

Power maybe your vacume is lower when you Come out of a curve?

1 way valve on booster has a leak and not holding vacume.

Under idle conditions lots of manifold vacume

Under heavy acceleration vacuum drops. Leak in 1 way valve drops vacuum in booster, drop off gas heavy on brake not enough time to recharge the booster

Manual. Only happens after hard turns.

 

[Dubob]

What rear axle are you running? What is the thrust like there? Maybe concentrating on the front is part of the equation but the back will also thrust in a turn and if the axle is moving enough to compress the pistons further could be contributing.

 

[67Dart273]

Check the hubs CAREFULLY when you pull the bearings. I've had two hubs in my live that somehow "ate" the bearing race loose inside the hub. You drive it in, the outer part seems tight. One of them, after cleaning the hub/ race of grease, was loose to the fingers after driven back in. It was not the race, it was the hub.

I don't remember them causing this problem, but it's another possibility.

For the record, I have never heard of this problem. I've had a number of rigs, back then, with disc brakes. I am 77 now. I have never been know for "easy stops."

 

[67Dart273]

What rear axle are you running? What is the thrust like there? Maybe concentrating on the front is part of the equation but the back will also thrust in a turn and if the axle is moving enough to compress the pistons further could be contributing.

How 'bout those SEA / SEE/ C clip axles????

 

[plymouth4onthefloor]

are these floater calipers or 4 pistons? if floater calipers, check if they touch the sway bar or suspension at full turn in, car on the floor!