Topic: Baffled by stuck rear brake - 1975 cb750f

[xallanx]

1975 CB750f.  I was on a ride and my rear brake got stuck gripping the disk.  The bike was immobile.  I wasn't far from home and got a tow.  Imagining that the pistons were jammed up by grime I took apart the calipers, soaked them in Berryman Chem Dip overnight, cleaned them out, replaced all rubbers, and replaced the brake pads.  Everything came together really nicely and I didn't find a ton of grime inside the calipers or on the pistons.  All was clean when I put it back together.  I bled the brakes and put new fluid in the line.  However, after brake fluid was replaced my brakes were locked up on the disk again!  I was frustrated, gave myself a break, and left the bike in the garage for a week or so.

Today I revisited the bike and the rear brake was no longer locked up on the disk and seemed to function correctly.  Before hitting the road again I wanted to get some feedback about my situation.

WHY DID MY REAR BRAKE SEIZE UP IN THE FIRST PLACE?
WHY DID MY REAR BRAKE SEIZE UP AFTER REAR BRAKE CALIPER REBUILD AND BLEED?
IS MY BIKE SAFE TO RIDE, OR SHOULD I TRY SOMETHING ELSE BEFORE I RIDE?

Thanks for the help!

[mrrch]

I know the fornt master cylinders have a tiny back bleed hole that when plugged causes the brakes to stay on. I'm sure the rear master has the same sort of bleed hole. i'm sure someone who has repaired one will chime in soon.

[MCRider]

I know the fornt master cylinders have a tiny back bleed hole that when plugged causes the brakes to stay on. I'm sure the rear master has the same sort of bleed hole. i'm sure someone who has repaired one will chime in soon.

yup. Almost for sure its the return hole in the master cylinder. When pressing on the pedal, you push fluid out of the reservoir to activate the pucks closing down on the disc. When you releasethe pedal, the pucks retract by various means, but only if they can displace that fluid back to reservoir. If the return hole is plugged, that fluid cannot return and the brake stays activated, sometimes, usually, locked up. If this happens you can gt the 10mm wrench out of your tool kit, loosen the bleeder valve to release the pressure. Then ride home without touching the rear brake.

You may have dislodged the plug when you bled the brake. It may stay that way or not. In a closed system you may wonder whats clogging that hole? Could be the inside lining of your hose is coming apart from age.

AS to safety, well if it locks up you come to a halt. That could be good or bad.

If you have dislodged the plug, I would run a quart of brake fluid thru it to make sure its washed clean. You can't really chase it any other way.

If it plugs again and stays that way, you remove the MC and go after it with a thin wire i the hole.

[w1sa]

+1 on the advice of previous posters......the thing about those air/pressure return holes is that they are so small and can be obstructed by floating debri but usually block off with age due to a build up of corrosion scale....when the hole finally refuses to allow pressure back bleed, the pistons will lock on.........this can happen rapidly with heat in the fluid from running........you apply the brakes solidly and they lock refusing to release.....often the engine power is insufficient to overcome the drag for immediate power/avoidance etc.

So, I would pull the master cylinder down and eliminate the possible hole blockage as a cause, and if thats not it, find the root problem...before using or depending on the rear brake.   

[BeSeeingYou]

+1 on the bleed hole.  You need to remove the hex bolt on that rectangular bit under the cup  to get a clear shot at the very tiny bleed hole and a larger one too.  You can't even see it if you look up into the piston bore once you have it apart.  Access from above is hard as the holes are at a right angle in a recessed area at the bottom of the bowl .  I would take it apart for cleaning and at that point get a OEM rebuild kit for around $45.  It should include all the cups, spring, checkvalve, piston, pushrod and boot.  The boot tends to collect grime and moisture over time so it's good to clean it out every now and then.  Get some new crush washers too if you order the rebuild kit.  This one is much easier to take apart than the front MC as access to the C clip is easy with or without a C clip pliers.

That first photo shows the piston stuck in the bore after sitting for 25 years.

[Tugboat]

+1 Good stuff above

[lucky]

The only other thing is does the caliper year and disc year measurements match?
Maybe the piston is traveling too far out of its bore and getting crooked in the bore.

Many owners are switching calipers and discs and I have not seen any measurements being given as to the fitment issues of the disk width and caliper.

[xallanx]

Thanks for all the responses.  Very helpful!