1150 RT Front Brake Master Cylinder plunger

[cris nitro]

OK, guys, I had a seperate thread going for my brake and clutch flush. Everything was going well, I flushed both front and rear wheel circuits, completed the rear control circuits and started on the front circut. While I was flushing/bleeding, the lever seemed to have less and less fluid "push" and more free play. It would come back periodically and I could continue pushing the old fluid down stream. However, at last the lever went 1/2 way and was hitting something solid. I stopped immediately and investigated. It seems that the hydraulic plunger had backed out of his little home and is binding with the lever. I can see something that looks like a plunger in the hole for the rear view mirror poking out which I can push back in with small screw driver. When I pull the brake lever, it pushes the "plunger" back into the mirror hole.

Also, when I was bleeding, I could see something moving in the little hole at the bottom of the resevoir when I pulled the brake lever. Now that something is no longer there.

 

Did I screw something up, or does this sound familar?

 

Maybe someone can shed some light on this and post a parts blow up of the front brake lever assembly.

 

Thanks and sorry for being such a pain for a simple task.

 

Cris

[dirtrider]

Evening Cris

 

Difficult to tell without seeing what you have but it sort of sounds like the cylinder piston is pushed into the master cylinder bore and sticking then what you might be feeling is the little lever push rod contacting the stuck piston..

 

OR, the piston is still OK and able to move but somehow the little push rod came out of the back of the piston and is now pointing at a place outside the master cylinder.

 

Maybe remove the brake lever (careful of the brake light switch) and see if you can determine what is happening. Is the push rod still entering the rear of the piston?

 

[AndyS]

Hi Cris, it sounds to me like the piston is sticking in the bore of the cylinder. When you started doing the bleeding, did you pull the lever fully back to the bars? This is normally what causes the problem due to that part of the bore not ever seeing the piston go in that far. as a consequence it can gall up there.

What happens next is that the piston (item 13-RS) jams in the bore, and the little actuating plunger (item 18) comes out of the housing and you can see this flopping around because it has come out of its rubber boot(item 15).

You may need to remove the lever (via bolt 18 - which is loctited in place), carefully remove the boot. Remove the circlip (item 14 - which can be a real PIA and even worse to install correctly), then get the piston with its spring out to examine everything.

Refitting is frustrating BUT BE PATIENT or you will damage something. You have to insert the piston (which is trying not to go in because its spring is pushing it back out again. Then you have to install the retaining circlip - at the same time as holding the piston in. The circlip is reasonably deep in a counterbore which makes instalation less easy. ENSURE it is correctly seated. Next the boot has to go correctly into its locating groove. Make sure it is fully seated all the way around its circumference if it isn't, dirt will get in. Finally the floppy plunger (item 18) which is screwed into the lever, has to be located through the boot and into the small locating recess in the end of the plunger (item 13). refit the lever pivot screw. check all works ok, then remove the pivot screw, lightly loctite it, and refit it.

Does this help?

[dirtrider]

Morning Andy

 

I think you have a mis-type in the above. You say-- (“You may need to remove the lever (via bolt 18”)

 

The lever is actually held in with pivot bolt 12 not 18.

 

While # 18 is removable, that is factory set to assure limited lever free play but still allow enough piston return to uncover the bleed hole or (take up port hole). There is no spec for re-setting that #18 correctly so unless he knows how it works and it’s intended function it can be reinstalled/re-adjusted incorrectly quite easily.

 

[AndyS]

Quite right DR, it should be bolt 12.

However if the item #18 is not disturbed it should be fine.

If you do remove item #18, there is a proceedure for resetting it. See attached.

 

Andy

[cris nitro]

Thanks, guys. This appears to be the problem. Yes, I did pull the lever to the bar only because the plunger was not returning after I pulled it 1/2 way, so I kept going a little deeper each time to get fluid push. I guess what was happening time was that the plunger was not returning and I kept driving it deeper.

 

Looks like I need to get in there and see what I can do.

Do I need to drain the resevoir before removing the plunger?

[dirtrider]

--Do I need to drain the resevoir before removing the plunger?

 

Morning Cris

 

Yes, definitely drain all the fluid you can as the minute you remove that piston any remaining fluid will run out the piston bore. Even after draining some remnants of fluid will come out that is trapped in the piston bore. Remember brake fluid is a fantastic paint remover so cover ALL painted surfaces including any plastic dash parts. Plastic garbage bags seem to work good as a paint/dash protection.

 

 

 

[cris nitro]

Any chance of pulling out the plunger to it's correct position without fully removing the rubber bootie and circlip?

[cris nitro]

Any chance of pulling out the plunger to it's correct position without fully removing the rubber bootie and circlip?

 

Or,,, can I put positive pressure on the plunger from the control bleeder, pushing the plunger back where it belongs? I'm scared of that circlip removal/replacment.

[dirtrider]

Afternoon Cris

 

Yes, you might be able to.

 

Maybe start by compressing the caliper pistons hopefully increasing hydraulic pressure enough to force the master cylinder piston back out.

 

Or maybe try removing the master cylinder lever then using a pair of snap ring pliers to grip the inside of the piston and pull it back out.

 

Maybe just try working the lever a little and get the piston moving back on it’s own.

 

I would bet you could blow it back using compressed air in the open master cylinder bleeder screw but that is real dangerous as there is a high probability of blowing brake fluid all over the bike and surrounding area.

 

In any case, any of the above will only move the piston back. If something is causing it to stick (like crud in the bore or rust in the bore) then just moving the piston back won’t cure that so there is a chance it could stick again at a later time maybe causing braking issues.

 

For future safety concerns you should probably address the REASON it is sticking while you are working with it now.

 

[NonComp]

In any case, any of the above will only move the piston back. If something is causing it to stick (like crud in the bore or rust in the bore) then just moving the piston back won’t cure that so there is a chance it could stick again at a later time maybe causing braking issues.

 

For future safety concerns you should probably address the REASON it is sticking while you are working with it now.

A couple of years ago I bought a '79 GL1000 that had sat for a long time with bad brake fluid in the system, causing corrosion. I used compressed air to push the single piston out of the front brake calipers. Warning: it takes very little air pressure to exert a surprising amount of force. I narrowly missed crushing my fingers between the piston and caliper. So turn your air pressure way down (a couple of PSI) if you try this.

 

Also, the master cylinder bore on that bike was corroded. I bought a piston kit and reservoir kit for it. What I found worked really well for cleaning the bore, was to wrap some fine steel wool around a wooden chopstick and then put the chopstick into the chuck of a cordless drill and ream out the bore with it. It removed the scale and cleaned up the bore sufficiently that the new piston worked smoothly.

[cris nitro]

I was able to take apart the throttle assembly apart today and removed the brake lever. The actuating plunger (#18) was in two pieces, one piece was the screw end in the lever with a ball socket on the other end and the other piece was the part that goes into the master cylinder bore and it had a ball end that had come out of the socket. I was able to push it back together, but after putting it all back together and starting to bleed the fluid, it came apart again. I am going to order a new plunger.

 

Thanks to all for their help and excellant pictures.

Cris

[AndyS]

Hi Chris, that wobbly plunger should be a 2 part component, but it shouldn't really have to come apart. Now that you have removed it you will DEFINATELY have to re set the 'blow by' (as per my attachment above).

As for piston. this really needs to be removed and examined. There is a resonable chance that the rubber seals on the plunger may have been damaged when the plunger was pushed in. Examine them carefully and if they are the slightest bit marked, replace them.

As for the bore of the master cylinder, I am not sure i would go with NonComp on his method for fettling the bore. Examine it carefully and buff out any muck. If there is pitting corrosion I would personally replace the housing with a good second hand unit.

 

WHATEVER, I suggest you disconnect the hose that connects to the master cylinder, so that during your cleaning proceedure, you don't send any contaminants into the brake hoses.

 

Remember too that brake fluid will strip paint and discolour other parts, be very careful and meticulous.

 

Remember too, that the item #18 needs to be loctited in place, but you will have to do that at the very final stage of setting the blow-by.

Initially get it all set up without the loctite on item 18. Once all the system is bled and working admirably, with the blow by set up.

Unscrew the item 18 outwards from the lever housing and as you do CAREFULLY count the number of turns. Apply a small dod of loctite and refit it exactly the same. double check everything carefully and quickly to ensure the blow by is working before the loctite cures, otherwise you will have to get the lever off and heat it up again to get the item 18 out again.

 

Andy

[cris nitro]

Thanks, Andy. I didn't take the plunger apart, it came apart on it's own. and I did not remove the screw end yet, but will have to, obviously to replace it.

Thanks for all your help. Will keep all posted.

[AndyS]

Thanks, Andy. I didn't take the plunger apart, it came apart on it's own. and I did not remove the screw end yet, but will have to, obviously to replace it.

Thanks for all your help. Will keep all posted.

 

Right, however, dont't get the grub screw with it's wobbly actuator (BMW pt no 32722333064) which you have called plunger, mixed up with the piston which is item 13 in the picture above and is BMW Pt no 32727658464.

 

It's this latter item that needs to get removed and checked. (along with the bore that it slides in).

[cris nitro]

I understand.

Thanks.

[cris nitro]

Thanks to all who helped me thru this. Over the weekend, I installed the new grub screw with it's wobbly actuator and finished bleeding all systems. The problem all along was the wobbly actuator kept coming out of the grub screw socket and when I would release the brake lever after bleeding, it would come apart. The new assembly stays together like it should and I do not have a problem with the piston hanging up in the bore. All is good.

Thanks,again.

Cris