Pad/Disk Contact @ 0 Pressure

Brian G
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Pad/Disk Contact @ 0 Pressure

Post by Brian G »

Hi Everyone -

I'd appreciate anyone's thoughts on an issue I'm having with the rear brake on my '79.

All was fine with the brake until I removed the wheel to have the local garage install a new rear tire. On the first ride after reinstalling the wheel, the brake began dragging so badly the pads were smoking and the disc discolored. I disassembled everything, dressed up the somewhat pitted caliper sleeve as best I could, fitted a new piston and seal, cleaned and greased the sliding pins and installed a new disk I happened to have. The m/c looked fine, but I was to discover the return port was blocked as she locked up again on the next test ride. Cleaned the m/c in an ultrasonic cleaner, ran a wire through the return hole and cursed myself for not doing it earlier.

So now I'm jumping at shadows. On the centerstand and spinning the wheel by hand, the brake applies then releases when I release hand pressure on the pedal, but after taking her for a two mile run without touching the pedal, the brake disk was warm to the touch. So this afternoon, I drained all the fluid from the system, took her for another two mile run and the disk still became warm to the touch (100 deg.F on my cheapo infrared thermometer).

So my question (finally got to it) is whether there is contact between the pads and disk in the 'unenergized' state and would there be some friction between the pads and disk even with no pressure in the system? A posting I read on a local sportbike forum insisted that light contact was normal, which, I assume, would generate some 'default' heat. Or might the caliper sleeve pits be causing the piston to hang up?

Thanks,
Brian
1969 BSA Rocket 3
1976 Kawasaki KH400
1977 Yamaha XS650D Cafe
1978 Honda CB550K
1979 Honda CBX

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herdygerdy
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Re: Pad/Disk Contact @ 0 Pressure

Post by herdygerdy »

It looks like you have nailed all 'the usual suspects' for dragging brakes except perhaps one.

I have worked on a few CBXs where the brake pedal pivot shaft was, err, 'as dry as a dead dingo's donger' and was not therefore not fully releasing the brake pedal all the way. It is possible when riding it that the pedal is not fully returning to the home and relaxed position, thereby keeping slight brake line pressure that keeps the pads rubbing on the disk and preventing full release.

You'll need to remove the RH aluminium footrest hanger bracket entirely so as to be able to remove, clean, and lubricate the brake pedal pivot shaft. Its a bit of a faff, but if you can do the brake caliper and m/cyl rebuild, then this will be easy-pezy for you. Let us know how it works out.

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Jeff Bennetts
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Re: Pad/Disk Contact @ 0 Pressure

Post by Jeff Bennetts »

Is the brake hose OEM, or old? The inside liner of the brake hose can deteriorate and cause a one-way flap, thus not releasing the pressure in the caliper.

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Re: Pad/Disk Contact @ 0 Pressure

Post by CBX-tras »

More than likely the caliper and m/c need rebuilding due to crud that forms behind the square shaped o-ring of the caliper.

I've seen them working just fine one day, lock up the caliper the next.

Fluid should also be flushed once a year.

Brian G
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Re: Pad/Disk Contact @ 0 Pressure

Post by Brian G »

Thanks for your thoughts, gentlemen.

There's a tiny bit of clearance at the pin in the master cylinder/brake rod joint; i.e. the actuating rod is fully extended so I'm pretty sure the m/c isn't holding pressure. At one point, before draining the fluid, I removed the caliper and squeezed the piston in until it bottomed, gently pressed and released the pedal by hand and saw the piston extend then retract (but not as far). The fact that there's still some heat generation with no fluid in the system leads me to believe that it's the position of the piston in the caliper or something else physical rather than hydraulic.

That said, it won't hurt to take it all apart again, clean everything again in the ultrasonic cleaner and replace the old reservoir hose (the brake line has already been replaced with a new braided item) just to be sure. I'll also fit another new piston seal in case the initial overheating affected its elasticity. I thought perhaps I'd missed something reinstalling the wheel or that the garage had bent something while installing the new tire but the disk appears to be running true at the center of the caliper holding bracket and I can see no excessive runout on the new disk. Then I guess the only thing left is that the pits in the caliper sleeve are preventing the piston from retracting sufficiently to clear the disk. Unless there's supposed to be a slight contact at rest and I'm worrying about nothing...

Anyone have a source for resleeving a pitted rear caliper? The local Honda shop says they've never heard of it being done.
1969 BSA Rocket 3
1976 Kawasaki KH400
1977 Yamaha XS650D Cafe
1978 Honda CB550K
1979 Honda CBX

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herdygerdy
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Re: Pad/Disk Contact @ 0 Pressure

Post by herdygerdy »

To clean out any crud & corrosion in the piston seal groove in the caliper, I got an old broken hacksaw blade and ground one edge down so as to leave a single tooth on one edge of the blade just a little less than the width, and a whisker more than the depth of the groove.

Used GENTLY, it makes cleaning out the crud buildup a breeze. Perhaps a similar result, but with less risk of damage to the alloy in the groove, could be achieved using a strip of hard plastic instead of the hacksaw blade, but the important cleaning 'tooth' may not be as durable.

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Jeff Bennetts
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Re: Pad/Disk Contact @ 0 Pressure

Post by Jeff Bennetts »

An inexpensive set of dental pics work well!

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Re: Pad/Disk Contact @ 0 Pressure

Post by daves79x »

And a Dremmel with stainless and brass brushes.

Dave

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Re: Pad/Disk Contact @ 0 Pressure

Post by Rick Pope »

A bent screwdriver works well, but my favorite is an old can opener, the kind that punches a triangular hole in the top of your beer can. You do remember those?
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Either garage is too small or we have too many bikes. Or Momma's car needs to go outside.

Brian G
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Re: Pad/Disk Contact @ 0 Pressure

Post by Brian G »

FWIW, I asked Apple Hydraulics in Calverton, NY for a quote on resleeving and Lazar pointed out (less than 6 hours after my e-mail!) that since, on the CBX, the seal is stationary and the critical surface is the piston, pitting in the sleeve is unlikely to be my problem. So it comes apart again for another thorough cleaning (hard to believe that after two cleaning sessions, I left enough crud in the groove to prevent proper piston retraction but as Sherlock Holmes said, "Once you have eliminated the impossible, whatever remains, however improbable, must be the truth" - or words to that effect). While it's apart, I should also measure the I.D. of the bore - maybe it's worn over limit and the piston is getting 'cocked'.
1969 BSA Rocket 3
1976 Kawasaki KH400
1977 Yamaha XS650D Cafe
1978 Honda CB550K
1979 Honda CBX

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Re: Pad/Disk Contact @ 0 Pressure

Post by daves79x »

Your caliper bore is not the problem, almost guaranteed. Never saw one that far out-of-spec.

Dave

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Re: Pad/Disk Contact @ 0 Pressure

Post by Rick Pope »

Agree with Dave. But, a pit in the piston might cause it to hang a bit, until it leaks. I've repaired pistons with JB Weld. Iknow it sounds hokey, but I tried it at the suggestion of Dave Ditner many years ago (25), and it's still on the bike.
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Re: Pad/Disk Contact @ 0 Pressure

Post by EMS »

A hydraulic disc brake does not have return springs, as we all know. But there is one design feature that helps return the piston from the back of the pad, thus relieving pressure when the lever is released.
It is the shape of the piston seal. The sealing surface of the seal ring is not flat, but has an edge on one side. This will make the seal "roll" ever so slightly during the forward movement of the piston when pressure is applied. When pressure is released, the force of the seal that wants to return to its normal shape will retract the piston and thus releasing pressure. I have seen calipers, where the seal was installed the wrong way around and they dragged more than usual.
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Brian G
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Re: Pad/Disk Contact @ 0 Pressure

Post by Brian G »

Dave and Rick - The piston was replaced after the first lock-up so it's relatively new and there are no leaks anywhere. Inspection after the second overheat showed no damage to the surface.

Maybe the garage did bend something installing the tire - now, even with no fluid in the system, I'm thinking I hear a slight intermittent sliding noise over the rattle of the chain when turning the wheel by hand!

I really do appreciate the comments - I'm totally flummoxed; this isn't rocket science! Perhaps I'll take her for a longer ride, bring along the correct allen key to remove the pads if necessary and see just how hot it gets. As I say, perhaps I'm just jumping at shadows and the disk temp. is normal and it won't get any warmer than 100 deg.F...

PS Just read EMS' post - now there's a direction I hadn't thought of! I'll flip the seal when I have it apart on the weekend. Thanks!
1969 BSA Rocket 3
1976 Kawasaki KH400
1977 Yamaha XS650D Cafe
1978 Honda CB550K
1979 Honda CBX

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Jeff Bennetts
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Re: Pad/Disk Contact @ 0 Pressure

Post by Jeff Bennetts »

Pad retraction in any caliper is totally reliant on seal 'pull-back'. Caliper seals are essentially square in cross section, it’s very rare to have tapered caliper seals that need to be directionally oriented, examine your seal with a jewelers glass, it’s most likely a square profile.

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