1976 Bronco Front Disk Brake Piston Action

[emathews61]

Happy Thursday!

I've attempted to poke around at previous threads but can't seem to find this exact answer.

I recently rebuilt the front end of my 1976. New axles, spindle, rotor, calipers, brake pads, steering linkage, etc. The only thing new I could not find was steering knuckles, but I did replace the ball joints.

I have some noise (ticking and rubbing), only at speed (~30 mph) coming from just the passenger side. It goes away when I press the brakes. If I lift up the front, the wheels spin pretty freely and I can't make any noise manually spinning it. However there is some drag from the brakes on both sides. When I installed the new rotor, calipers, brake pads it was really tight.

Finally coming to my question. Should the caliper piston recess completely away from the brake pads when I lift up on the brake pedal (sucked in by the reverse hydraulic pressure)? Currently it does not and again there is some drag. Might be a problem?

Any advice would be appreciated.

Thank you,

Eugene

 

[Broncobowsher]

Slight drag is normal. In normal driving it is microscopically gapped after a few dozen revolutions.

There is a phenomenon known as "knock back" that happens to road racers who make multiple hard turns without being on the brakes. The rotors and spindles flex slightly and knock the pads back slightly and there is a long stroke on the first apply afterwards. Not a normal thing to get. Takes a lot of cyclical lateral high loads on the knuckle. That gets you a gap, that is bad.

 

[Apogee]

Best guess is that your pads are maybe on the high side with respect to their thickness tolerance, as that can make initial assembly a bit tight and can lead to excessive drag, not that I think you have excessive drag. If you can rotate the rotors by hand without a wheel installed and you're not seeing excessive heat when driving, then it sounds pretty normal to me. The square o-ring seals inside the calipers do retract the pistons slightly upon release, but we're talking thousandths of an inch. Any runout of the rotors will also serve to push the pads/pistons back, but this is generally less than .004" in a properly maintained system.

As for the ticking sound, does it sound rotational in time with the tire or more random like with road surface or something else?

 

[ntsqd]

First thought on reading one side only noise that goes away when pressing on the brake pedal is that one of the pads, usually an outer one, isn't well retained to the caliper and is rattling. Really common to have this occur with the GM calipers due to the way that those pads are retained.

 

[emathews61]

Best guess is that your pads are maybe on the high side with respect to their thickness tolerance, as that can make initial assembly a bit tight and can lead to excessive drag, not that I think you have excessive drag. If you can rotate the rotors by hand without a wheel installed and you're not seeing excessive heat when driving, then it sounds pretty normal to me. The square o-ring seals inside the calipers do retract the pistons slightly upon release, but we're talking thousandths of an inch. Any runout of the rotors will also serve to push the pads/pistons back, but this is generally less than .004" in a properly maintained system.

As for the ticking sound, does it sound rotational in time with the tire or more random like with road surface or something else?

Yes I can rotate the rotor without the wheel and the ticking is rotational in time with the tire. I'm going to rotate the wheels today and see if that makes any difference.

 

[emathews61]

First thought on reading one side only noise that goes away when pressing on the brake pedal is that one of the pads, usually an outer one, isn't well retained to the caliper and is rattling. Really common to have this occur with the GM calipers due to the way that those pads are retained.

Thank you. Yes I will pull the caliper today and check the clip.

 

[emathews61]

Slight drag is normal. In normal driving it is microscopically gapped after a few dozen revolutions.

There is a phenomenon known as "knock back" that happens to road racers who make multiple hard turns without being on the brakes. The rotors and spindles flex slightly and knock the pads back slightly and there is a long stroke on the first apply afterwards. Not a normal thing to get. Takes a lot of cyclical lateral high loads on the knuckle. That gets you a gap, that is bad.

Thank you. Good to hear. I replaced the brake booster also and I know that if the stem is too long it can cause issues. I will disconnect from master cylinder and see if it eases up any.