Topic: Back brake nearly caught fire......1980 Honda cb900c Custom

[Hondawggie]

I bought and installed a used-but-working rear master cylinder onto my 1980 Honda cb900c.
I bled the brake and the brake pedal tightened up and air bubbles coming out of the bleed nipple were reduced to no bubbles.

I then got ready to take her out for a spin.   While I pushed the bike out into the driveway to have enough room to climb aboard and start her, I noticed a bit more drag on the bike than I had felt when pushing the bike around in the days before I fixed and bled the back brake.

However, I said to myself "well, I have been pushing the bike around these past few days and the rear caliper has been completely removed during that time, so today after re-installing the caliper, of COURSE it's going to feel like there's a bit more drag.  Plus I just bled the back brake.  I'm 100% convinced that's all I'm feeling here."

The amount of drag while I pushed the bike on the driveway was *minimal* -- not enough to impede me from moving this heavy bike around.

I then started the bike, drove down the road, used the back brake a couple times, WHOA -- lots of drag on the rear brake!  I only got 1 block from my house and turned around.

The rear brake was smoking like nearly on fire when I got her back home -- only reason I kept driving it with the rear brake sticking was --- I *knew* that if I didn't use the 900cc of power to overcome the dragging rear brake, I would have not been able to push it home.

I immediately got my fire extinguisher out of the garage -- luckily, no flames erupted.

The smell of the smoke was:  the disc pads too hot.

Now just *what* might allow me to successfully bleed the brake, but the puck (apparently) in the caliper will not retract?  This is a real simple caliper -- only one puck.

Could this dragging be caused by the master cylinder I just bought and installed?  That would be odd, because I was able to bleed the rear brake in 5 minutes -- despite the fact that the brake hose was full of air, and the reservoir and the master cylinder were also full of air, as was the caliper.  The master cylinder seems to be functioning -- at least as far as pushing the puck against the brake pads and the rotor.


Before I bled the system today, I had *completely* disassembled and cleaned the caliper, removed that internal o-ring and cleaned the groove, then wiped it all down with rubbing alcohol, which evaporates quickly.

Then, before re-installing that internal O-ring in the caliper, I slid the puck in-and-out of the caliper after rubbing some brake fluid on the outside of the puck and on the inside of the caliper bore.  The puck slid in-and-out just fine.

ANY IDEAS?   I will need to let it cool down for a while before I can get at it.

[MCRider]

There are two (at least) functions to the brake letting go of the disc, when pedal pressure is released. #1 the puck/piston is withdrawn from the disc by the flexing of the oring you replaced and B. the fluid so displacedby the withdrawing of the puck has somewhere to go, like back into the resorvoir.

If A is working then #2 is not. Fluid is not traveling back into the MC reservoir. There is a small return hole that is likely plugged.

And yes the MC will bleed out properly with this condition. Very common on fronts. And it may not be severe, it doesn't take a lot of pressure for the brake to overheat. When it happened to my front, I pulled in on the lever, and the brakes never retracted and the bike came to a halt. I got out the trusty tool kit, turned a fitting, pick one any one, and released the fluid pressure and rode it home.

Do that to your rear, release the pressure but DON'T touch the brake pedal again. If all's well, its the return hole in the MC.

[BobbyR]

I do not have a rear disc but after fixing a flat tire a friend a friend adjusted my rear brake a bit too tight. I dragged it 60 miles at 70 mph and it was smoking also. Here is the bad news, it cooked the rear cush rubber drive to a point they were melting, The grease in the wheel bearing poured out like cooking oil a day later. If I were you I would disassemble the rear wheel and see if any damage was done. A rear wheel lockup is no joke even at low speed.

[Hondawggie]

I do not have a rear disc but after fixing a flat tire a friend a friend adjusted my rear brake a bit too tight. I dragged it 60 miles at 70 mph and it was smoking also. Here is the bad news, it cooked the rear cush rubber drive to a point they were melting, The grease in the wheel bearing poured out like cooking oil a day later. If I were you I would disassemble the rear wheel and see if any damage was done. A rear wheel lockup is no joke even at low speed.

I only drove one city block then back so the distance was not great, and the very worst part of the dragging did not commence right away so the exposure to the rubber cush drive was brief.  But your point is well taken!   I never even thought about the rubber cush drive -- the squeezing of the cush drive's rubber must have been intense even though the duration in my case was brief (about 2-3 minutes).  So yep I'm going to have a look at that.

That MC, I figured there might be something I wasn't aware of here.  I will loosen a hose fitting and see if the puck retracts.  Then I'll have to figure out how to clean the m/c.

Here's the FSM's diagram of my cb900c's rear brake -- would this blocked hole be found in the fluid reservoir?   Or in the master cylinder that has the internal piston?

[BobbyR]

I do not have a rear disc but after fixing a flat tire a friend a friend adjusted my rear brake a bit too tight. I dragged it 60 miles at 70 mph and it was smoking also. Here is the bad news, it cooked the rear cush rubber drive to a point they were melting, The grease in the wheel bearing poured out like cooking oil a day later. If I were you I would disassemble the rear wheel and see if any damage was done. A rear wheel lockup is no joke even at low speed.

I only drove one city block then back so the distance was not great, and the very worst part of the dragging did not commence right away so the exposure to the rubber cush drive was brief.  But your point is well taken!   I never even thought about the rubber cush drive -- the squeezing of the cush drive's rubber must have been intense even though the duration in my case was brief (about 2-3 minutes).  So yep I'm going to have a look at that.

That MC, I figured there might be something I wasn't aware of here.  I will loosen a hose fitting and see if the puck retracts.  Then I'll have to figure out how to clean the m/c.

Here's the FSM's diagram of my cb900c's rear brake -- would this blocked hole be found in the fluid reservoir?   Or in the master cylinder that has the internal piston?

It is not the strain on rubber but the heat you need to be worried about. With a Disc outboard of the hub the rubber could be just fine, I would look at the bearings. You probably just burned the brake compound.

[Hondawggie]

There are two (at least) functions to the brake letting go of the disc, when pedal pressure is released. #1 the puck/piston is withdrawn from the disc by the flexing of the oring you replaced and B. the fluid so displacedby the withdrawing of the puck has somewhere to go, like back into the resorvoir.

If A is working then #2 is not. Fluid is not traveling back into the MC reservoir. There is a small return hole that is likely plugged.

And yes the MC will bleed out properly with this condition. Very common on fronts. And it may not be severe, it doesn't take a lot of pressure for the brake to overheat. When it happened to my front, I pulled in on the lever, and the brakes never retracted and the bike came to a halt. I got out the trusty tool kit, turned a fitting, pick one any one, and released the fluid pressure and rode it home.

Do that to your rear, release the pressure but DON'T touch the brake pedal again. If all's well, its the return hole in the MC.

MCR you hit the bullseye on this one, when I went to the bike just now, its on the centerstand, back wheel off the ground but the back wheel is stuck solid.

I loosened the brake hose right at the caliper for a second and the back wheel come free.  So you're right, the back pressure is not being relieved, it's staying in the caliper and the puck cannot retract.

In your mind, should I disassemble the part of the mastercylinder's piston is, or is that back-pressure-relief-hole, the one that's blocked, would that be in the fluid reservoir?

Or is the blocked-up hole inside the master cylinder's piston and shaft piece?

[BobbyR]

Try opening the holes in the MC first

[MCRider]

Try opening the holes in the MC first

Ding ding ding    

It should not require disassembling the MC. I'm not familiar with what that cylinder looks like, but the fronts can be cleaned by removing the reservoir top, and poking a small wire thru the fluid and into the hole. But for specifics you'll need someone who has seen it.

[Hondawggie]

Try opening the holes in the MC first

By the way, that's the best-looking santa Ive seen on the board here.  Do you mean the holes in the bottom of the fluid reservoir?  The actual m/c part is not attached to the reservoir cup.  

A rubber hose connects the master cylinder (that's the part with the piston inside) to the master cylinder brake fluid reservoir cup. You can see that in the diagram I posted above.

It sounds like in your mind and MCR's too that its best to start by poking a wire into holes in the bottom of the brake fluid reservoir cup and only mess with the m/c piston housing later if need be?

[MCRider]

Yeah the return should be in the reservoir, as that's where the returned fluid ends up. If you can see holes in the bottom of the reservoir tank, one feeds fluid into the system as you depress the pedal, the other allows it back into the reservoir upon release of the pedal. Return should be the small one, if you have the choice.  Pretty small, I've heard guys using guitar string wire, single strands of picture hanging braided wire, hardware sometimes carries small gauge stainless wire. Get one that fits and poke it into the hole.

I've heard sometimes there is/was so much gunk in the reservoir that the return hole was no longer visible.

[Hondawggie]

I found 2 holes, one much smaller than the other, on the body of the m/c piston shaft piece -- the 2 holes are behind a flange where the brake fluid drops in from the fluid reservoir that hangs above the piston shaft piece.

I stripped the paper off one of them twist-ties you see on a kitchen garbage bag liner to hold it shut -- the wire inside was slender enough to fit into the small hole.   It felt like there was no hole-through but I kept pushing on the tie-wrap wire and the tiny hole give way and opened up.

I reassembled everything and bled the system but the back wheel is still sticking after I use the rear brake.   To get the back wheel loose again all I have to do is loose the brake hose connection at the caliper for 1 second and the back wheel is free.

So this time, I removed EVERYTHING:  the rear caliper, the brake hose from the caliper to the master cylinder, the master cylinder, the hose leading from the master cylinder to the brake fluid reservoir, then the reservoir itself.

I'm going to completely disassemble the master cylinder and clean it, and do the same the the caliper (for the 2nd time).

Then I'll reassemble and bench-test it off the bike.  Only way to be sure at this point.

My expectation is that once I clean it, put it back together then bleed it -- I should be able to cause the brake puck to extend out of the caliper body, then I should be able to push the puck back into the caliper body.  If I can't push the puck back in after all that, I'm not sure what to say at that point.

[MCRider]

You're on the right track, it'll be fine. If you're disassembling the master, you can dry it out and get some air in there and it'll be good as new. Pesky things.

And the thing is, even if you got it to work from dislodging some debris, the debris is still in the system. So much better to break it down.

[BobbyR]

I found 2 holes, one much smaller than the other, on the body of the m/c piston shaft piece -- the 2 holes are behind a flange where the brake fluid drops in from the fluid reservoir that hangs above the piston shaft piece.

I stripped the paper off one of them twist-ties you see on a kitchen garbage bag liner to hold it shut -- the wire inside was slender enough to fit into the small hole.   It felt like there was no hole-through but I kept pushing on the tie-wrap wire and the tiny hole give way and opened up.

I reassembled everything and bled the system but the back wheel is still sticking after I use the rear brake.   To get the back wheel loose again all I have to do is loose the brake hose connection at the caliper for 1 second and the back wheel is free.

So this time, I removed EVERYTHING:  the rear caliper, the brake hose from the caliper to the master cylinder, the master cylinder, the hose leading from the master cylinder to the brake fluid reservoir, then the reservoir itself.

I'm going to completely disassemble the master cylinder and clean it, and do the same the the caliper (for the 2nd time).

Then I'll reassemble and bench-test it off the bike.  Only way to be sure at this point.

My expectation is that once I clean it, put it back together then bleed it -- I should be able to cause the brake puck to extend out of the caliper body, then I should be able to push the puck back into the caliper body.  If I can't push the puck back in after all that, I'm not sure what to say at that point.

You can run some denatured alcohol thru the system and see what crud comes out. It dissolves the crap and does not harm the parts.

[Hondawggie]

You're on the right track, it'll be fine. If you're disassembling the master, you can dry it out and get some air in there and it'll be good as new. Pesky things.

And the thing is, even if you got it to work from dislodging some debris, the debris is still in the system. So much better to break it down.

With help from you folks I have learned a lot here.  Thanks for the input and guidance.

I used my 'inside snap ring' pliers and removed the snap ring at the end of the master cylinder and pulled the m/c piston and springs out.

I found about three little black specs of what appeared to be hardened dirt.  The biggest one was about 1/4 the size of the head of a pin, the other 2 specs were smaller.  I used air to blow out the m/c bore then reassembled.

I then made sure the brake hose was clear -- it was fine.

The brake hose being disconnected from the rear caliper, I took the caliper in my hands then tried to squeeze the brake puck back into the caliper body to make sure the puck wasn't binding -- the puck easily slid back into the caliper bore  -- and brake fluid shot out all over the end of the furniture in the house, instantly made me think of my ex, she was not around to comment on my hobby in this case.  Sometimes it is cool being divorced and this was one case,  brake fluid clean up yet no #$%*ing 'take it outside'  no we do motorcycles *inside* the house sometimes.

So I did not disassemble and clean the caliper -- it's fine.

Put the hose, the m/c, and caliper back on the bike -- bled it - ITS FINE NOW.

So if someone had asked me "Can 2 or 3 specs of crusty dirt, about 1/2 the size of the head of a pin in total -- require a 900cc, 85hp motor to overcome?"  I would have laughed, now I know better.   Those specs were blocking the fluid return hole in the m/c and locking up the back brake to the point of a near-fire situation.

Here's a picture of the tiny 'fluid return' hole in the m/c that caused all the problems -- a very thin wire from a garbage bag tie-wrap is all that will fit in the tiny hole -- the larger-looking hole around the wire is reduced just out of site of the photo to the diameter of that wire:

[MCRider]

Great! The tie wrap wire was very creative, it is a small hole, and it doesn't take much to plug it.

[rmarvit]

the two holes for input and output of the fluid from rear mc can they be accessed by usings the wire through the reservour hole?

[BobbyR]

Yes, you only need to remove the cap, suck out the fluid. Run the strands of wire through those two holes at the bottom.  Scrub them out good. Gently squeeze the handle like 1/16 to let the fluid flush out the crap you loosened. Put some fluid in the reservoir and suck it out to remove any suspended dirt and then refill and bleed if needed.   

[scottly]

Bobby, I believe he's asking about a REAR master, with a remote reservoir like the cb900?
rmarvit, if this is the case, you need to remove the plastic fitting at the end of the reservoir hose from the MC body. It has 2 screws attaching it, and pulls straight out. This will expose the holes as shown in the previous pictures.

[BobbyR]

I stand corrected. Thanks