Hi, I had a problem a few months ago with the rear brake on my K100RS locking up, the heat wrote off a disk, caliper and pads, had this fixed and all was well until yesterday when the same thing happenned
after riding about 10 miles, I pulled up in time to avoid the same amount of damage this time but the wheel was locked. After about 1/4 of an hour the brake had cooled and i was able to turn the rear wheel and ride home using only the front brake. Has anyone an idea what the problem might be ?

i had the same problem. the previous owner had just replaced the same items. but within days i experienced the same problem.the pressure equalizing hole in the master cylinder was plugged.there is one hose going to cylinder but two holes to internal part of cylinder.
probably time for an overhaul and fresh fluids.GOOD LUCK!

See if you can get your master cylinder ultrasonically cleaned. Easiest way or you need to see that you get to the little holes.........

also on an added note the brake hose may have collapsed internally
definatly worth checking before ordering parts

Thanks for the tips guys, I'll check these parts out as you suggest

Paul

had the same problem before, time for a overhaul
the rubber seal on the piston that brings it back gets old and worn and wont bring the piston back.

Sorry Mate, the spring brings it back, not the seal.

Was just testing everyones knowledge on the braking system

Off course

Reviving this topic.
My rear brake is still a piece of poo.
New pads, new master cylinder kit and after 5km it locks on.
I have to open the bleed nipple to relieve the pressure to get the wheel to spin again.
Also the brakes do not work much at all.
I can lock the wheel on the stand in 3rd but in 2nd or 1st with my foot hard down it just keeps turning.

Revived this topic as I saw the comment about the return holes for the fluid. the pressure equalizing hole in the master cylinder was plugged.there is one hose going to cylinder but two holes to internal part of cylinder.
Are these in the cylinder or the piston (the one in the cylinder).
I have checked the caliper pistons and they both move freely.
Brakes bled properly. No air.
I really don't know where to go from here. Ultra sonic clean of master cylinder?

If its locking on like you say then you probably dont have enough slack in the adjuster screw and its holding the piston down the bore a tiny bit and preventing the proper fluid return.
As for not working well check the alignment of disc and caliper, vertical and horizontal if the pads are contacting at an angle then its not good news for stopping. Check the disc for true it should be no more than .003inch

double check the rubber flex hose going to calliper from master cylinder it may be leaking internally or slowly disintegrating internally
also the dust seal around the calliper is this free of dirt etc causing the piston to stick out and make sure the pads are loose in the calliper else they can stick as the disk gets warm

Done, done and done. Thanks toto. I actually had the caliper completely apart to clean and repaint as I was doing the rest of the bike.
On other advice I did not replace the seals in the calipers, but they move free and easy.
Checked the hose to the caliper and there are no bulges or leaks.
When locked on and I open the bleed nipple they instantly return to the 'proper' place. ie the wheel spins free.
Both calipers move in and out. Checked it off the bike.

+1.....on the adjuster screw.

Sorry Rick, I did not see your post. I did play with the adjuster screw but I have backed it off so far now there is almost no more adjustment left.
I will back it right off and take it for a test ride and adjust it from there.
I have not checked alignment of the pads. I did add a bit of tension to the spring that goes over the pads and under the retaining pins just in case they were a bit loose.
I will check the disc for 'true' and see how it goes.
I have the little return spring attached also to the pedal.

just a thought guys ...

if the bore of the master cylinder was slightly corroded ....and ...the piston was not returning properly to the second relief (bleed hole) to allow the thing to work properly ,,i would expect some binding .....does this make sence ....

by contrast if the front master cylinder resevoir is over filled and there is not enough air volume in the resevoir to allow the bleed valve to evacuate ...resulting in little or no presure to the fronts ...as the returning action of release doesnt let the pump work correctly .

you may have to pull the piston on the master cylinder on the rear and give the bore a bit of a clean up rossco ...this will also let you clean up those widdle holes to let the thing breath (pump) a little better or at least the action of the piston to go full travel ....i think that many of us have noticed over time that the rubbers build up resistance ...or more likely a bit of an edge in the bore restricts the full travel ...i wouldnt be attempting to push the master piston up the bore or pull it out any further than it wants to go ...as usually the rubber will be distorted and even tear the seal off the leading edge ...resulting in verry poor performance ...even leaking from the from seal washer on over extension and corroded parts passing through the now hardened rubber bits

i would blame water absorbtion over time for this phenomenom

food for thought ?

Does your bike have ABS? The previous owner of mine had the same problem with the rear brake locking on when used and eventhough there are no faults when the ABS modulator was bypassed the rear brake works fine. He had a braided stainless steel line made to go directly from the master cylinder to the caliper.

Ditto on the rear brake adjuster being out of adjustment, just went thru the same thing on my K75.I had it adjusted in too far and it was not letting the piston in the rear master return far enough.

Looks like I am in for a full strip down and clean.
Just rode 2.5km to work.
The brake adjuster is at minimum and there is a 1/4 inch gap between it and the plunger.
I used the brake as you normally would on the way to work (which was about 10 times) and when I arrived I could not turn the rear wheel by hand.
The little return spring is in place.

I would say the problem is the master cylinder not returning to home position so there may be a different reason. The piston only has to move about 2mm to close the fluid return hole before it puts pressure to the caliper. Was the master cylinder resleeved or did you just put a kit through it or just clean it.

I cleaned it and put a kit in it.

I will pull the lot apart this weekend and give it a good going over.
Took me forever to bleed the damn thing so maybe all that pumping has made something hang in there.
It will give me something to do in the garage while I watch The Great Race

clean it .....? how did you do that rossco ...

often a refinish of the bore of the cylinder needs care ....

what i did on the front master was to get a suitable hone .....in my case a piece of dowel sufficiently undersized and some wet and dry that would bind in a fine cut allong the length of the dowel . basically a lengthways split

then wrapped some fine wet and dry around the dowel in the correct direction to leave exposed the wet and dry to the bore .

a cordless drill was employed to do the lynishing process with the dowl ...luckily i was able to get a good polish along the bore ,,,without going too big on the whole process ...leaving a smooth and unpitted result ...new piston and return spring with new rubber was installed after a clean out with metho amd a de-clag of all the venting and pump assembly holes ...amazing amounts of small crud came out ...and i had noticed a fine edge /mark ...(probably the normal return position of the rubber on the piston in the cylinder bore ) was removed in this process .

the new end seal went in with some fresh brake fluid to wet it all up around the seals and working parts ..(dont try to assemble dry at all )....and haleluya it all sealed up properly ...that was 10 months ago (round about ) and no leaks etc since ....i guess i got lucky ,,eh ?

have a few verry clean rags (cotton is best and uncontaminated ) and lots of metho on hand mate ...along with some fine wrapping wire to clean out the widdle holes etc

good luck !

Why you'd be widdling on your discs I'll never know.,....

Good point Charlie it could be the fine crud in the "widdle" holes causing the brake lock up
but as usual I assumed it has been thoroughly cleaned inside before fitting new seals may pay to flush the line as well incase the crud is still floating in suspension

just spent about 4 hours cleaning and flushing with metho.
Then pulled apart again and put some wet n dry on a hot glue gun stick (was the best fit i had) and turned it by hand inside the master cylinder. and flushed again with metho.
Pulled the caliper apart and removed the pistons, cleaned, oiled with brake fluid and re-assembled.
I tried to use a pressure bleeder, but it would not pump the fluid through? Problem?
Bled the brakes and they are exactly the same.

I am going to buy a new master cylinder and never EVER going to pull it apart.
Not happy Jan.
Tossed up wether to spend the extra on the whole master cylinder or just the kit.
looks like I made the wrong choice.
At least I have cold beer

That's a bugger....but as you say, you've got cold pig's ears and clearly a view of cars going around and around in ever decreasing circles at Bathurst.

bugga !!!

did you find the valve holes in the master cylinder , and then clean them out ? how was the bore ?

sad to hear that it didnt pan out ol mate .

I could see the grub screw hole and the off center back hole and the inlet hole. Are there more? There was a small recess in the middle of the cylinder at the rear and I assumed it was the end of the drill from when it was originally made.
Don't tell me that is some sort of one way valve?
The bore was good to start with. Clean and smooth. That is why I just rubbed a bit of wet n dry through by hand

rossco wrote:I could see the grub screw hole and the off center back hole and the inlet hole. Are there more?

It should be two holes at the inlet, a real thiny one deepest in the bore of the two................it seems like this one is clogged on yours.

The first seal have it`s resting position between these two holes.

Didn`t you have some problem with the plastic fitting for the inlet tube some time back, could that have something to do with the problem?

Inge K. wrote:

rossco wrote:I could see the grub screw hole and the off center back hole and the inlet hole. Are there more?

It should be two holes at the inlet, a real thiny one deepest in the bore of the two................it seems like this one is clogged on yours.

Didn`t you have some problem with the plastic fitting for the inlet tube some time back, could that have something to do with the problem?

I did have a minor leak at the inlet tube but a bit of a twist and it has not leaked since.

Can I unscrew the hose off the back of the cylinder and clean it from there?
I have not taken that part off before.

I don`t think it would be any easier to acces from the rear.

I would have removed the MC from the bike to the work bench, much easier to do a proper job.

About the plastic fitting....I have never removed one of these, but as you say that you twisted it a bit...it could be that it`s just press fitted with a o-ring.

If that could be removed, you would have a very good acces.

i went off looking for a cross section view rossco (diagram) to explain ...but nothing found

i agree with inge ...there should be a hole down in the guts that the first seal runs over between off and on

i feel sure that this hole blocks up and is the reason we see so many give up ....and just go buy a new master

i think its a tiny hole that would fill with rubber and gunk over time ...and to the eye would be hard to distinguish , likely to be part of the reason for hard bleeding process as well .

wish i could exlain better

there maybe a chance to blow it out with compressed air ...with the cylinder apart ....and being able to block the rear most hole (to allow full pressure to the smaller hiding one up front ) ...maybe soaking with acetone could help clean out the paths ....but not sure of the reaction to aluminium and other important stuff in there .



btw ...those last few laps at bathurst were fantastic eh ?

I will tell you now from experience that you will be very unlikely to get it out in a useable condition. It has 3 or 4 fins all the way round and when it is inserted they fold back a bit so that when you try to get it out it wont come, bit like barbs on a fish hook its a oneway effort.
I had a master cylinder resleeved and a new one was fitted but I dont know where it comes from or from what manufacturer. You may be able to get one from some place that does the resleeving.
There is one from a Mazda master cylinder that fits the hole sort of but it has a bigger fitting for the resevoir tube.

Thanks Rick, then we know that the plastic fitting can`t be removed.

charlie99 wrote:i went off looking for a cross section view rossco (diagram) to explain ...but nothing found

Here is a car tandem master, could help a bit...............

which also explains why the rear calipper sends smoke signals, after owners have been messing with the adjuster screw. (without enough knowledge)

Guess ultrasonic cleaning might be the only way to get it done. These new MC's are bloody expensive!

could be exactly that what is needed rene ....good thought



just so folks dont have to open another window ...inges link

Image 1 is the hole coming from the reservoir. I can only see one hole here.
Image 2 is the end of the cylinder. I can only see one hole here.
there is no photo of the grub screw hole. no point

Is there a hole I am missing? they are all clear and clean. Air and fluid all blown through them in both directions.
If I remove the piston in the master and fill the reservoir, fluid flows easily from the reservoir, through the cylinder and out the nipple on the caliper at the other end so there are no blockages in the holes I have found.

That picture you posted Inge suggests there should be a second hole from the reservoir further down the bore, but I cannot see one.

Rick, I was not going to pull the plastic cap on your earlier advice. I gave it a twist about 1/8 turn and it settled nicely back into place and does not leak.

I have ordered a new cylinder from motorworks. Guess I have till about 3pm tomorrow to find the answer and cancel the order or just bite the bullet and replace the whole thing.

rossco

charlie99 wrote:

btw ...those last few laps at bathurst were fantastic eh ?

Some of the best ever. I am a Holden fan but kind of hoping Reynolds could pull it off as it was a fantastic comeback.
Worst part of the race was staying sober in the hope I got the brake fixed and could go for a ride.

Have a look/read HERE for a bit of info on our system, hope it helps.

aww that sucks rossco (staying sober ) i cant talk ...was the same , hoping to get gerty on the road before dark ...it aint going to happen ....so i relented and went down to the bottle shop for some takeaways ...

(screen to clean up ...lamps to fitt under and repaint ..and glue drying )

maybe the equalisation hole is the issue ..should be a little closer to the pushrod end of the bore ....this is the one that would stop the return of the piston easily ...and thus keeping the caliper locked up ? just a thought as you cant see 2 holes .

Stick about 100psi into the top of the M/C and see if it blasts the crap out or maybe get a bit of wire with a right angle bend and feel round for the hole.

Rick G wrote:Stick about 100psi into the top of the M/C and see if it blasts the crap out or maybe get a bit of wire with a right angle bend and feel round for the hole.

That was my last attempt.
Removed the reservoir and stuck 120psi down the hose. Held my thumb over the end of the cylinder and blasted it.
Stuck the nozzle in the end of the cylinder and blasted (didnt do much as it wasn't sealed)
Stuck the nozzle on the nipple on the inlet side of the caliper (caliper was off) and blasted air back.
Re-assembled, bled and just the same, though I have not tested for it sticking on as it is a bit late now to go out and I have caught up on the beers is missed during the day.

New cylinder and fingers crossed. Had not planned to spend $177 on the bike this weekend
thanks for the help everyone.
I am defeated.

you have tried ol mate ....commiserations .

Not that good picture....but the two ports ending up at each side of the inner seal is visible.

Inge K. wrote:Not that good picture....but the two ports ending up at each side of the inner seal is visible.

Inge, the only thing I can see remotely like a second return hole is a ring down near the base, but this looks too far away to be a return near the inlet.
When blowing air in it was not obviously coming out of more than one location.
I do not know the history of this cylinder so I guess anything could have happened to it.
On Blakey's suggestion I will stuff the thing in my bag and bring it to Bingara for the guys to ooohhh and aaahhh over and then tell me what I have done wrong.
rossco

here we go ....workshop day at bingara ...!!

may as well ,,,rt brought his temp sensor that measured good (to alaluen )

stan is bringing his pockets lock , for rekeying, already

and a few other trades will be happening i guess

what we need rossco is a primus valve cleaner a little flat looking item with a piece of fine springsteel probe on the end at right angles to the tool ..

ill have a look around .. havent seen one for ages ,

charlie99 wrote:

what we need rossco is a primus valve cleaner a little flat looking item with a piece of fine springsteel probe on the end at right angles to the tool ..

ill have a look around .. havent seen one for ages ,

You better hope I don't also order the Beaver kit before I leave or we may have to open a shop front to get it all fixed.

lol ..

no worries rossco ...just make sure you bring it with you .