Topic: Front Brake on '76 CB750K Will Not Release

[run-tmc]

Hi everyone,

I've spent more than a fair amount of time purging the rust, dirt and air out of my front disc brake and struggled to get any pressure on the front lever. I left it alone for awhile, came back and was able to build up nice pressure for the first time - no sponginess at all. My problem is that once the disc is locked onto the brake, I am unable to disengage the brake and release the rotor again. Has anyone experienced similar symptoms? Any thoughts on the most likely causes? My dad recommended that I pull the caliper, then use a fine emery cloth to polish the exterior of the piston. Does that seem like a good spot to start? Thanks guys!


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[run-tmc]

*brake


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[Powderman]

Did it just occur suddenly or has the bike sat for a long period of time? Disassemble the caliper and inspect the parts before sanding anything. Pull the seal and inspect it and also the seal groove for any debris or oxidation/corrosion build up in the groove. If there is crud in the groove it will make the seal tight against the piston causing excessive pressure on it. Scotchbrite pad might be better than emery cloth unless it is very fine. If that doesn't cure the issue look to inspect the master cylinder for the same issues. If it's a floating caliper make sure the parts all move freely and are lubed properly so it can spring back from the rotor when pressure is released

[Powderman]

*brake


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You can go back and "modify"your post to  correct the spelling error.

[bryanj]

There are multiple posts about sticking brakes if you search. most common problems in order are

(1) Siezing pivot pin in caliper swing arm
(2) Corrosion behind the caliper piston seal and/or corroded piston
(3) master cylinder return hole clogged
(4) clogged or collapsing flexible lines

[ekpent]

Like the guys have said if you have not cleaned the caliper yet that is a must do for starters. Even if the piston is still working somewhat the outside edge of the brake pad may have a lot of corrosion and won't retract.
   Take a rubber mallet/hammer and give the caliper a few good whacks and see if that loosens the wheel/brake up.
  If and when you take it apart be careful not to strip out the heads on those 2 allen bolts as they can be a little fragile and sometimes very tight, go to Autozone or someplace and get a nice fresh bit that fits nice and tight that fits on a socket wrench in case you need the ole' breaker bar.

[crazypj]

You have to do the complete system. Pivot, master cyl, caliper.
Quick search will give you way more information than you ever thought possible

[MCRider]

All good ideas. My test would be: release some fluid pressure either at a banjo bolt or the caliper valve. IF the brake releases and tire moves freely, the most likely problem, and very commonly covered here, is the pressure return hole in the master cylinder.

[run-tmc]

Thanks for all the feedback guys. I'll admit that my search for sticking brakes wasnt very thorough. I'll try out a few suggestions as soon as possible and get back to the board with results.


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[ekpent]

Whack it(caliper) good with the rubber mallet a few times first and see if it rolls again 

[xnoahx]

I was having this problem on my 78.  I went through the caliper and cleaned it all up but and still had the problem.  For me replacing the master cylinder did the trick.  You could probably just get a rebuild kit for yours though.  The earlier years are easier to get those for.

[run-tmc]

Here is an update on my sticking front brake issue:
I pulled the front caliper apart and the attached pictures reveal the corrosion/scoring on the inside of the caliper. Once I hydraulically pop the piston out, it does not go back into the caliper without a ton of pressure from my thumbs, making me think that seal is pushing out too much toward the piston. A few questions for the experts now:

1. Has anyone tried a spring-loaded honing tip on a handheld drill to lightly clear away the scoring in the caliper? Any other good cleaning methods for caliper?

2. Once the seal is removed, should I plan to have a new seal ready for install? Has anyone had success reusing an old seal? Any recommendations on where to order from if new is the only way to go? Any tips for correct install?

As always, thanks for the support guys!
 
 

[MCRider]

My 2¢: You want the caliper as clean as can be, but honing is not necessary. Just wipe vigorously with a cleaner, non-abrasive. Pick at the grove with a pointy object to clean the corners. Like an ice pick that's been bent. I use a cotter pin puller. Seals are available all the usual places, on line, etc. I use motogrid.com.

Its most likely the master cylinder. Once the caliper is clean and reassembled the movement of the piston from engaged to withdrawn is about 5 to 8thou, Hardly noticeable. They are seldom a problem.

The MC, on the other hand, is notorious for acting up. Even if you think it is clear.

OCICBW

[flybox1]

dont sand the piston.  it needs a smooth finish to seal well against the piston seal.
dont hone the piston bore...honda made them the right size already.
install a new piston seal.

1. ensure the fluid return hole int eh MC cup can let fluid back in.
2. with a small mirror, get a look a the piston seal groove.  de-gunk it.  it needs to be clean for the piston to properly ride on the seal.
3. remove any paint from the metal brake pad backer so the pad itself wont lock inside the upper bore of the caliper (too think paint here...a common theme with aftermarket pads)

[ekpent]

A soft wire wheel on the end of a Dremel will make quick work of cleaning the seal groove.

[MCRider]

A soft wire wheel on the end of a Dremel will make quick work of cleaning the seal groove.

Good idea!