Replaced brake pads - at correct torque it binds

[nickw9815]

Howdy,

I replaced my brake pads on my 6a. I fully returned the master cylinder all of the way. However, when I torqued the caliper bolts down all the way, the brakes grab the disk and do not spin. My mechanic told me this is common, and the solution is to slightly back off torque so it can spin.

Has anyone experienced this? Are there spacers I can get to make the caliper slightly wider and not grab the disk?

 

[Roadjunkie1]

Howdy,

I replaced my brake pads on my 6a. I fully returned the master cylinder all of the way. However, when I torqued the caliper bolts down all the way, the brakes grab the disk and do not spin. My mechanic told me this is common, and the solution is to slightly back off torque so it can spin.

Has anyone experienced this? Are there spacers I can get to make the caliper slightly wider and not grab the disk?

I think you meant that you had fully returned the slave cylinder (the one at the brake caliper side of things). The puck should be pretty much flush with the housing. I have squeezed them to where I thought they should be, and the pads didn't fit quite right. I compressed the pucks of the slave more (I was surprised they moved further!) and the pads went on fine. Watch the reservoir as it can overflow as the puck is compressed. There will be a little drag that you can hear with movement of the wheel, but not locked like you are describing. I've also not heard of the bolts being loosened up to allow the wheel/brake disk to spin. But I am surprised on, sometimes, a daily basis.....

 

[jabarr]

Do NOT back off the torque!!! If pads are properly fitted and puck is fully retracted, add a shim between the caliper half’s. There is a part number for the shims but they’re expensive. Best to just make it out of .020 aluminum.

 

[planenutz]

Do NOT back off the torque!!! If pads are properly fitted and puck is fully retracted, add a shim between the caliper half’s. There is a part number for the shims but they’re expensive. Best to just make it out of .020 aluminum.

Yep - what he said.

Check to make sure you've used the correct pads too, as you will find that some pads from different manufacturers will physically fit the calipers however they might be slightly different dimensions. The spacers are a common method to overcome the issue and you can remove them later once things have settled down.

 

[recapen]

What brand of brakes and what brand of replacement pads?
I have replaced the pads on my Clevelands a few times using new pads from AIrcraft Spruce without this issue.
I have the upgraded rotor kit that included slave cylinder spacers to account for the thicker rotors - but it uses the original pads.

 

[Taltruda]

I can almost guarantee that the disks are warped. You can lay a straight edge across them and see that they get cupped. I like the Mcfarlane APS ones. The A model planes tend to get heated more than the TW planes.. in the meanwhile, you can try making a shim from a cardboard cereal box, if that’s not enough, then transfer it to some thin aluminum.. but eventually you’ll need me disks.

 

[sansoneservices]

Howdy,

I replaced my brake pads on my 6a. I fully returned the master cylinder all of the way. However, when I torqued the caliper bolts down all the way, the brakes grab the disk and do not spin. My mechanic told me this is common, and the solution is to slightly back off torque so it can spin.

Has anyone experienced this? Are there spacers I can get to make the caliper slightly wider and not grab the disk?

IF, IF, the caliper bolts have drilled heads then yes you can back off the bolts a maximum of 1/8 of a turn. Safety wire will prevent them from backing out further. After a minor amount of braking you will be able to torque the bolts without binding. I prefer a thin shim be installed

 

[nickw9815]

I ended up making a shim out of 0.25mm stainless steel.

 

[Ralph Inkster]

I think you meant that you had fully returned the slave cylinder (the one at the brake caliper side of things). The puck should be pretty much flush with the housing. I have squeezed them to where I thought they should be, and the pads didn't fit quite right. I compressed the pucks of the slave more (I was surprised they moved further!) and the pads went on fine. Watch the reservoir as it can overflow as the puck is compressed. There will be a little drag that you can hear with movement of the wheel, but not locked like you are describing. I've also not heard of the bolts being loosened up to allow the wheel/brake disk to spin. But I am surprised on, sometimes, a daily basis.....

I would try this first.
Use a C clamp or large pair of water pump pliers to gently retract the piston fully into the caliper housing. If this doesn’t loosen the grip on the rotor, than try a shim.

 

[nickw9815]

Yeah, to be clear I used a clamp to get the cylinder sub flush. Even then it still binded. Took nearly a full turn on the bolt to make it not bind, so I added 2x 0.25mm spacers.

Good call out on the rotor being warped. I’ll look into it.

 

[Taltruda]

I wouldn’t use a clamp to retract the piston.. it goes in with thumb pressure. It’s not a ford F250.. a for the shim, I would remake it out of aluminum. Stainless next to aluminum seems to cause the aluminum to corrode..

 

[Roadjunkie1]

I wouldn’t use a clamp to retract the piston.. it goes in with thumb pressure. It’s not a ford F250.. a for the shim, I would remake it out of aluminum. Stainless next to aluminum seems to cause the aluminum to corrode..

Agree with the thumb pressure rather than a clamp. Unless the clamp is small and used with finesse. A paper/cardboard shim was mentioned. The brakes can get hot enough to flame this up. Or at least toast it. Stainless will cause aluminum to corrode so, if you use that, make sure you remove it once the brake pads have trimmed themselves down enough to torque the bolts down.

It has been debated whether, with the bolts snugged down, they really need to be safety wired. I have always wired them as that was what I was taught. Looking at other aircraft, most seem to be wired, some not. Mine: always wired. What are those holes in the bolt head for if not for a little wire going through.......?

 

[Ralph Inkster]

I wouldn’t use a clamp to retract the piston.. it goes in with thumb pressure. It’s not a ford F250.. a for the shim, I would remake it out of aluminum. Stainless next to aluminum seems to cause the aluminum to corrode..

The c clamp is part of my process for changing pads. I had a plane puke all over my hangar floor one time when it popped its piston so made it a practice to secure them ever since.
& yes, I use a bigger clamp on my truck.

 

[jrs14855]

Rather than use a c clamp it might be better to remove the piston, clean up the bore of the cylinder with fine scotchbright and replace the o ring. I have never had to use a shim following this procedure.

 

[nickw9815]

Good call out on the galvanic corrosion. I used stainless shim material since it was handy, but I will remove the shims after a few flights when the pads wear down slightly.

I also used an Irwin clamp, not a C clamp. The clamp force of these is quite low.

 

[gasman]

Something is not right. With clean guide pins and new pads (Cleveland or rapco) properly installed, the caliper should rattle over the disc before pressing on the pedal to set the piston. You need to find the problem and solve it without the shim.

The disc is not new, there should be plenty of clearance.

 

[Taltruda]

Something is not right. With clean guide pins and new pads (Cleveland or rapco) properly installed, the caliper should rattle over the disc before pressing on the pedal to set the piston. You need to find the problem and solve it without the shim.

The disc is not new, there should be plenty of clearance.

The problem is most likely a warped and cupped disk. If he lays a straight edge across it, it’ll be apparent. I can almost guarantee it. It’s an A model, they tend to get brake abuse and warped disks. Wearing the pads to fit is a weak fix at best. APS164-01700 and your brakes will be good again. Perhaps it is a good time to do the VANS brake SL to change the caliper orings to Viton and use Royco 782 high temp fluid

 

[gasman]

If the brakes are set up properly, there is plenty of room before seating the pads for the first time, for even a warped disc to rotate without binding.