Topic: Incomplete Front Caliper Retraction (CB500)

[sammermpc]

The front brake works fine — it just drags. It's not actually a huge issue, and I can ride w/ it easily enough, but if I put my hand down on the front disk, it feels warm/hot to the touch. If I left the front wheel, it definitely drags, and there's no way I have the .006" clearance. I've got basically none. What I have is "kinda softly dragging" and "braking real good," though I can go between the two easily enough .

What I've done:

• Replaced the piston w/ new aftermarket
• Replaced piston seal
• Cleaned out seal groove with a dremel
• Rebuilt MC w/ MC kit
• Bled using some variety of the many techniques posted
• Verified my fender is mounted right (in front of) the caliper
• Greased and checked for free-play in caliper swingarm

I replaced the piston even though based on what I saw online, it didn't look that pitted, because I saw some fluid leaking out in the seal as I pumped it in and out. I mean a real tiny amount, just a bit of wetness. Now that I've replaced the piston, I still see that very slight leak! I wonder if I need to replace the caliper housing. It is weathered. I saw someone post about having their housing re-honed in a shop.

Does anyone have any more details on that? Any other suggestions of obvious places to look for a solution/ My brake lines are pretty old. I could still have a few air bubbles hanging out..?

[05c50]

Assuming everything that you've done has been done correctly, here's two more things that I can think of. It's possible that the old brake lines are restricted and not allowing the pressure to fully release. Or the tiny return hole (I think it's called a spooge hole) is plugged and not allowing fluid to flow freely back into the reservoir.

.......Paul

[ekpent]

Assuming everything that you've done has been done correctly, here's two more things that I can think of. It's possible that the old brake lines are restricted and not allowing the pressure to fully release. Or the tiny return hole (I think it's called a spooge hole) is plugged and not allowing fluid to flow freely back into the reservoir.

.......Paul

Those are very good suggestions and also the brake pad itself could be sticking in the caliper.

[Rocky2010]

Had the same issue, what finally fixed it was putting grease on the caliper around where the brake pad moves in and out. Its working great now well the brakes are as good as they can be on an old bike 

[TwoTired]

You put grease in with glycol based brake fluid?!!?

[sammermpc]

I just got some of the recommended Dow Corning High Vacuum Grease in the mail, so I'll put that on the edges of the pad and the exposed parts of the piston, next time I disassemble per the suggestions I've seen (not on the seal, etc.). I've sanded down the pad and everything so it moves freely (when I turn the caliper upside down, it drops out).

One thing I did notice with the new piston, is that it's VERY tight. With the old piston, I could slide it in and out with my fingers fairly easily. With the new one (and the new piston seal), I couldn't at all.

The thing I notice, though, is that when I'm pumping the piston, it always moves further out than it does back in. How does that work, exactly?

Yeah, the return hole seems fine. I've seen a lot of the posts about cleaning it up, but despite the age the reservoir is pretty clean.  I've cleaned it up though.

[flybox1]

What I have is "kinda softly dragging" and "braking real good,"

This is what I have with my dual fronts, and i'm happy with it..

[TwoTired]

One thing I did notice with the new piston, is that it's VERY tight. With the old piston, I could slide it in and out with my fingers fairly easily. With the new one (and the new piston seal), I couldn't at all.

It matters if the seal is lubed or not, about how easy it slides. (more later).  Without the seal, the piston should move freely inside the caliper.

The square section seal sits in grove with a slanted pitch base.  This forces the seal into a trapezoidal shape when assembled.   The seal distorts when the piston moves outward.  When when fluid pressure allows, the seal tries to return to original shape, it pulls the piston back with it.   If the pad still has the nylon ring and is bathed in silicone lube, the piston movement also pulls the brake pad back as well.
You might now understand that grease of any kind is NOT desired on seal to piston contact area.  Brake fluid or Brake Parts Assembly Lube (BPAL) is the proper substance to place at that point, with BPAL being highly preferred for its resistance to absorb water.

[sammermpc]

So is the actual deformation of the seal itself the only thing determines contraction? I wonder if my issue is actually just the pad (which does have the nylon pad, but no silicone), not being pulled back with the piston. When I push on the caliper with my hand, though, I don't feel any play between the pad and the piston face.

I have never used anything other than brake fluid on the seal or the piston itself. I read over your instructions about lubing w/ vacuum grease, and I'll do that and see how things look.

[TwoTired]

So is the actual deformation of the seal itself the only thing determines contraction?

You'll notice a spring on the caliper arm where you adjust the .006" gap on the "stationary" side.
The seal pulls the piston/pad away on the caliper side, and spring pulls the "stationary" pad away on the opposite side.  A bit of road vibration helps this all function better.  However, if the wheel bearings are worn, the rotor won't stay centered, and side loads will force contact with the brake pads.

I wonder if my issue is actually just the pad (which does have the nylon pad, but no silicone), not being pulled back with the piston. When I push on the caliper with my hand, though, I don't feel any play between the pad and the piston face.

Not sure you can feel 0.006".

I have never used anything other than brake fluid on the seal or the piston itself. I read over your instructions about lubing w/ vacuum grease, and I'll do that and see how things look.

I'll reiterate that you don't use grease on the seal, or parts of the piston in contact with brake fluid.  That would be where assembly lube is best employed, as it won't draw water out of the air or water splashed on or behind the brake pad (like brake fluid will).   The Dow Corning silicone grease is used like paint to keep water away from metal and thwart corrosion.  It will not dilute or combine with water or air, and will not creep, liquify, or migrate to the friction pad/ rotor interface.  It's lubricating properties are NOT what is useful or required in this application.

[sammermpc]

Just a quick update, here. I re-bled the caliper and got a few more bubbles out, and the brake feels significantly firmer, so that was definitely part of the issue. I also re-adjusted with a .006" feeler gauge in place (you're right — it's tighter than I had realized!), and re-assembled using vacuum grease in the appropriate places.

All together, while the front wheel is far from free spinning (it probably can make about 1.25 revs while suspended), and the disk heats up during normal riding, I think that's about as good as I can get things at the moment. Perhaps I'll consider new brake lines or something in the future. It sure would be satisfying to have a 100% free-spinning front wheel!

[MCRider]

Just a quick update, here. I re-bled the caliper and got a few more bubbles out, and the brake feels significantly firmer, so that was definitely part of the issue. I also re-adjusted with a .006" feeler gauge in place (you're right — it's tighter than I had realized!), and re-assembled using vacuum grease in the appropriate places.

All together, while the front wheel is far from free spinning (it probably can make about 1.25 revs while suspended), and the disk heats up during normal riding, I think that's about as good as I can get things at the moment. Perhaps I'll consider new brake lines or something in the future. It sure would be satisfying to have a 100% free-spinning front wheel!

You likely won't reach that goal. The pads at 6thou, will always drag on the rotor making a shushing sound. AS TT mentiones, road vibration helps it all.

I recently had mine off and I judge it to be in proper condition and 1.25 rot is what I got. I do have dual discs.

[calj737]

Also verify the caliper mounting bracket is serviced. The rotating pin needs to be super clean, then greased for full free-play

[MCRider]

Also verify the caliper mounting bracket is serviced. The rotating pin needs to be super clean, then greased for full free-play

Real good point.

[sammermpc]

Also verify the caliper mounting bracket is serviced. The rotating pin needs to be super clean, then greased for full free-play

Yep! That's one of the first things I did. It was fairly stiff, initially, but now moves very nicely (at least when it has some place to move).

At any rate, I've got some other fish to fry, with weeping top-end gasket and a spark (#4) that keeps on dying on me, but that's something for another thread! I am looking forward to my first top-end rebuild this winter.