rear axle play

[chpr1972]

A few months ago I towed my car on a car dolly about 25 miles . When I got back home I had no brakes. I called Wilwood and ask about axle play and what was max spec for play. They told me there was no spec and I should have not lost the brakes.
A couple of days ago I wanted to scare my granddaughter and I was doing a u turn and dropped the hammer with the wheels turned to finish the turn. When I went to use the brakes I had pedal drop of about 1/2 the pedal travel. Like before it pumped back up with a couple of pumps. Any Ideas?

[Motobrewmaster]

This sounds allot like pad knock back to me. What setup are you running for your back brakes? More info will certainly help as there are many very experienced people on here to help, they just need to know the details.

[chpr1972]

It's a ford 9" with wilwood 12.88 rotors and 4 piston calipers. It has a parking brake in the rotor hat. It was in the car when I bought it so I do not who made it. It is in a 66 Lemans.

[Motobrewmaster]

I personally do not have any experience with Ford 9" rears. However a simple test would be to put the rear axle up on jacks and disengage the parking brake. Now check to see if you have any side to side play at the wheel. If you have any side to side this could be causing pad knockback.

Sounds like you have the same rear brake setup as me. I have some pad knockback on mine. However this is due to me running a 12 bolt with C-clips which always have a little side play. I am working on this to minimize it until I can replace the rearend with a full floater setup.

[GEARBOXGARAGE]

As Motobrewmaster mentioned above with his 12 bolt/C-Clip rear end, it is more common in those designs to get pad knock-back from end play. With the 9" having pressed bearings and retainers, there shouldn't be any. However, the axle flanges may be flexing just enough that it is causing a bit of this. We typically put a check valve inline on the rear line going to the back. Most time the blue 2# check valve will be enough to minimize this effect without any adverse effects on pad pressure/wear. Here's the link to the Wilwood unit: http://www.wilwood.com/MasterCylinders/MasterCylinderProd.aspx?itemno=260-13706
Typically they advertise them for set-ups where the Master is mounted low in the chassis, such as the old pickup trucks, to prevent flow back and loss of pressure in the system, which in theory, is what is happening in hard turning with the pad knock back. A short while ago I came across a thread posted either on here or over at 67-72chevytrucks.com where they put a 10# check valve in and did some testing and recording of data, such as retained pedal feel, rotor temps, etc. If I can find the post I'll share it here.

[parsonsj]

It's pad knockback.

Solution is to eliminate the wheel's ability to push the axle in or out. There are 3 basic solutions: 1. Use a floating caliper or a floating rotor. 2. Convert your live axle to a floating axle. 3. Convert your rear suspension to an IRS.

If you want to stay with your live axle setup, there are a series of tweaks or tunes you can do to reduce axle movement. Check and see if you have Set20 (tapered) wheel bearings and what kind of retainer you have. If you have sealed roller axle bearings, that is probably the worst offender.

Once you do all the mechanical fixes, then you can turn to hydraulic fixes like check valves and the like. But in the end, you'll have to live with some knockback unless you turn to one of the 3 solutions noted above. Some guys that road race such cars get in the habit of pumping the brake pedal during straights so they have brakes when they get to the turn. :)

There are dozens of threads here about knockback. Do a search...

[Skip Fix]

A few things to check on rear itself on end play -how much there is to start with. If a C clip there are thicker C clips. It can also be a symptoms of worn posi clutches, worn axle C clip button, worn cross shaft buttons ride on- that can be shimmed up or a new posi.

[chpr1972]

I will try a 2 lb check valve.
Gearboxgarage, Do you remember if they had excessive pad wear? That is what I would worry about with a 10 lb valve.
It is a big ford roller bearing so is there a way to shim it? It is kinda worry some when you loose that much pedal. What would have happened if I had done a full turn one way and then reversed direction and went the other way around. That is basically what happen when I used the tow dolly. I had to pump it several times to pump up psi.

[GEARBOXGARAGE]

I don't think they recorded pad wear, but I do remember they would record rotor temps. I should also mention that the test was performed on a GM 12 bolt rear end, so a C-clip set-up. On our truck, it also has the GM 12 bolt. The 2# valve has worked well so far on twisting roads, but we are starting to plan out phase 2 of the truck, and most likely a 9" will eventually find its way under the bed. Ideally, a full floating variety, but we'll see when the time comes.

parsonsj also pointed out some good info as well. The floating calipers with a 2# valve would probably be the "best bang for your buck" but upgrading mechanical components will provide the greatest success.

I was able to find the thread I had mentioned early, here's the link: https://www.pro-touring.com/threads/101080-Soft-Pedal-vs-Axle-Runout-solutions-Willwoods-(fixed-callipers)-on-a-12-bolt-GM

At post 36 is where the conversation starts about using the check valves.

[Skip Fix]

Why GM on the 79-81 TAs used a floating caliper on C clip 10 bolts.

[Skip Fix]

My 78 TA NEW had so much free play when I hit a left turn I could here rubbing. They never figured it out put on sway bar bushings- I do think rebuilt posi. When I did the first brake job (Velvetouch semi metallic shoes) I could see the brake drum had been shoved into the backing plate!

[badazz81z28]

I have tapered bearings on my 9" and the axles do not move at all. If there is pad knock back, it's so minor you would not see any effects.

[rrelco]

its pad knock back when using a fixed caliper thats whats going to happen, i couldn't afford a floater set up so what i did after i broke my axle i called Dutchman axles and had them make my axles .030 longer the pad knock back is gone. i also added a 10psi residual valve

[Paraman1]

its pad knock back when using a fixed caliper thats whats going to happen, i couldn't afford a floater set up so what i did after i broke my axle i called Dutchman axles and had them make my axles .030 longer the pad knock back is gone. i also added a 10psi residual valve

Did you install the 10 PSI residual valve prior to the new axles ? If so did it help much ?

I see most manufacturers recommend a 2 PSI for disk brakes and a 10 PSI for drum and am curious as to what the downside to running the higher PSI valve might be .

Also curious how you measured your axle play to come up with the extra .030 in length and how much axle play you had after installing them ?

[rrelco]

See PM

[chpr1972]

I ask the forum quite a while back about loosing brakes after I towed my car on a car dolly or when spinning in circle. Friday I pulled an axle to repair an axle oil leak. My axle had moved enough to hit the seal. The rear end was already in the car when I bought it so I had no information on who made it.
I called the 9" experts at Curries and they identified my rear end as a Moser. The man at Curries told me to check the retaining plate as the lip goes in towards the bearing, not towards the axle hub. I checked mine and sure enough the lip was to the outside. He told me that the retaining plate with the lip to the outside would have about 1/4" play and that is what mine has.
So Monday I will buy another set of bearing and take it down to the guy that put the bearings on about 3 years ago and see if he will press the bearings off and on for free. If not I'm taking it to a different shop.

[chpr1972]

I am very happy to report that my knock back is completely gone! My brake petal is higher and I use less brake petal to stop. I went to a safe place and spun the car around completely in a circle and lost no petal travel. To me the brakes seem to perform even better.

[parsonsj]

Glad to hear it. Knock back on live axle cars is nearly always caused by axle movement.