Hey all, I’m helping out a mate who’s having problems with his front brakes. Ok, it’s an 84 boxer, but the brake system is very much the same as a K bike with identical parts used. 
The problem is that the front brake pads start to tighten up against both rotors while riding with very little use of the brake lever and takes about 10 minutes to occur. The bike will be forced to a standstill but releasing the banjo at the mc resulting in a squirt of brake fluid fixes the problem for the rest of the ride, only to reappear the next day. Also, if left to cool down the callipers will release their grip on the rotors,  it will reappear later. Note that BOTH rotors are hot indicating (in my mind) that the problem lies in the master cylinder. Work so far includes a master cylinder rebuild kit, both callipers have been stripped with new seal kits installed, and the bike runs braided lines albeit the lines were fitted some years ago. The calliper pistons are bores are all good. The only thing left is the master cylinder itself although it seemed fine when I fitted the new piston with no scouring. The return hole to the reservoir is clear. What have I missed? All help and advice much appreciated.

A few things, since I have a 1983 K100RS and a 1984 RT and more recently also a 1984 Sprint that started out as a naked K100.

When I got the 83 RS it had sat for 17 years so needed to do the obvious. Brake hoses, caliper overhaul, master cylinder overhaul.

Having lots of parts for the 84 RT I thought simples. Not so. Transpired the 83 was different from an 84K. So we are told there are early K100 and later K100. Not so, there is super early, 83 RS did have same calipers as the boxer but mounted opposite way around so left on the boxer is right hand on the K100, but they were rapidly changed to a bigger diameter piston which is what you will be given for the early caliper that takes the 55mm pad. The later ones take the 57mm sintered pads. There were other niggles too, so in the end I just replaced the front and rear master cylinders and left the repair kits I had back in my shed. Before I did that I noticed the master cylinder also had some minor differences. It is possible your repair kit is not allowing the pressure to be released after using the brakes. Are the brake hoses replaced?

Now another thing I had recently on my 83 when I was changing the pads....I bought 55mm pads for it. But they did not fit properly. Since I have done all the work myself I knew something was wrong and its a bunch of pads that the backing plate is marginally bigger than 55mm [I have a few Ks and buy in bulk]. I had to grind them down to get them to fit freely. What was happening was they were jamming inside the caliper and not retracting. Both left and right discs affected. Symptoms exactly as you describe...if I used the brakes, coming from cold, pads would not retract properly. Then friction heated them up so that they expanded and increased the braking effect and of course heating it even more. But it also heated the fluid significantly which might overwhelm the return. The system relies on very little pressure to release the brakes and will never release them if there is the slightest restriction on the pads returning to their rest position.

It sounds like the liner in the brake line from the master cylinder is collapsing and needs to be replaced.  It is also possible that there is a bunch of crap in the line and is blocking the movement of fluid.  I had the same problem with one of my bikes.



About pads sticking, I have found that there are two different pads used during the run of K bikes and one is  about 1mm larger than the other.  They look the same and unless you measure them you will never see the difference.

Point-Seven-five wrote:It sounds like the liner in the brake line from the master cylinder is collapsing and needs to be replaced.  It is also possible that there is a bunch of crap in the line and is blocking the movement of fluid.  I had the same problem with one of my bikes.



About pads sticking, I have found that there are two different pads used during the run of K bikes and one is  about 1mm larger than the other.  They look the same and unless you measure them you will never see the difference.

The reason I asked about the brake hoses is exactly the point you mention, they break up and will block holes like this. If replaced, the brakes need to be bled on the side stand too.

You are right on the brake pads sizes, the difference is 2mm, 55mm and 57mm. The 55mm pads are normally organic/green/non sintered and only these fit the early calipers. The 57mm pads are also sintered, so they are wider for the later calipers and will only fit in them. You can buy these as green/organic too, but they dont fit early calipers.

But the pads I ordered were sold as 55mm pads and labelled 55mm, for the early calipers and were made slightly too large for them so they were sticking. I just ground the sides back a little to fit.

One problem I have found over the years is that some aftermarket(non OEM) pads are made with old punch press tooling that is worn out.  Worn out tooling makes oversize punchings that will need tweaking to fit.

Thank you both very much for your in outstanding informative advice. As mentioned in my opening post the lines have all been replaced and these are stainless aftermarket items so should be good, however it did occur to me that these might be the problem. I’ll chat with the owner and see when the pads were fitted.

Spent some time on my mates R series today and have discovered the following. The pads were a little tight so ran the grinder down the sides to make movement easier however when the pistons were pushed out a little via a squeeze of the front lever, they would not retract when weapon of choice was inserted between the pads in the usual manner to retract them. This was only overcome by cracking a banjo resulting in the immediate retraction of the pads. So something is preventing brake fluid from travelling back up the lines to the master cylinder. The banjo on the same calliper we were working on was cracked so that’s a no brainer that the pads retracted easily so the next step is to repeat the exercise but crack the banjo at the master cylinder and see if the same results occur. If so, then the problem lies in the master cylinder - which has a new piston kit in it.  Please note that  ALL lines are braided stainless and less than five years old. 
Here’s a question - there is a modification with one line from the master cylinder to the right calliper and then a line over the mudguard to the left calliper with one bleed nipple on the left calliper to bleed both callipers. Not perfect I know. So, each calliper has two nipples, on the right calliper the feed from the mc enters the calliper via the outside nipple, and the crossover line runs from inside nipple to inside nipple on the left calliper with bleed nipple on the outside. Confused? Me too, but does it matter? There are no one way valves within the calliper so shouldn’t matter but ya never know… To add to the issue, I asked when this problem first occurred and he said after we installed the calliper overhaul kit. Great, but prior to that the brakes were so ineffective they were dangerous, so overhauling them has simply revealed an issue that was possibly previously hidden. Any thoughts?

That's a setup I've seen before but I class it as 'non ideal', mostly because it makes bleeding the brakes a RRPITA. To me it sounds as if you have bled the brakes correctly and I'm going for a problem in the master cylinder. Specifically, it wouldn't surprise me if you found that one of the o-rings got pinched on reassembly and it's holding the piston in place.

Interesting Dai, I’m at the point of going back into the master cylinder anyway and seeing what’s going on. As mentioned previously, I’ll try cracking the mc banjo and if that allows the pads to retract immediately then that’ll confirm the issue is there and not further down the lines.

It might be worth spending a little time flushing the lines with the banjo bolts disconnected at the calipers and draining into a container.  A pint of fluid pumped through would confirm whether or not the problem is debris in the lines or something else.  If it is debris, it should flush it out.  Also, the resistance you will feel will tell you how bad the lines are plugged if they are.  Flushing with the banjos disconnected should have little lever resistance. 

From what  know about master cylinders, I am doubting that is where your problem is.

KiwiK100 wrote:Here’s a question - there is a modification with one line from the master cylinder to the right calliper and then a line over the mudguard to the left calliper with one bleed nipple on the left calliper to bleed both callipers. . . . Not perfect I know

Dai wrote:That's a setup I've seen before but I class it as 'non ideal', mostly because it makes bleeding the brakes a RRPITA.

It seems that was a factory assembly on late model Bricks. Images of 1992 Bricks show that setup on Standard, S and RT models. On my '95 Standard, the brake line runs directly to the right caliper passing through a fender-mounted wire keeper on the way. The rear half of the fender has a notch on each side for the bridge line to pass through; a split grommet surrounds the pipe and the grommet perimeter is grooved to nest it into the notch's edges.

It might seem to be a RRPITA (PITA in the USA) but all I do is open the left bleed screw and flush with ≈60mm of fluid. Of course, I use a piston retractor which some might consider a PITA (R, RR or non-R) also, but that's life. The brakes on my Brick have been functioning well on the flats and on the mountain roads here for 65 thousand miles; I don't take it to the track.

I had that setup on my K75RT and outside of having the bleeder on the opposite side from the brake lever I found bleeding to be pretty easy and wish I had the same setup on my K100RS.  No problem at all pushing old fluid and air out of the front calipers.

My 1987 rescue bike had front brake problems. Flushed a!! Lines, used a litre of dot4. Problem was the rubber boot in the master cylinder disintegrated and the bits clogged the lines. Only about 40 percent of the boot was visible when the top of the master cylinder removed the 60 percent was in bits blocking the lines! Working good now. Got 200 km on bike since refit. Hope this helps someone.

I normally vacuum bleed my brakes with a home made setup. However I saw a one way valve brake bleeder online for around $3.50 Au and purchased three. One has gone to a mate and the other in stock. I pumped all the fluid out of the calipers using it and then removed both calipers and cleaned and painted them. After refitting them fitted the one way valve cracked the bleeder and bled them by just pumping the lever. The process was easy and quick and I had no problems with air pockets at all. I was impressed with how easily it worked, easier and quicker than the vacuum bleeder. During the cleaning process the sealant around the bleed nipples was replaced with PTFE pipe sealant. The plastic hose supplied to go over the bleed nipple was too small so a plastic hose was slid over a short piece of it to upsize it. I had previously looked at speed bleeders which are expensive in OZ $ 50.00 Au a pair plus freight and hard to get.
Regards Martin.

Ignore the valve core tools.

Martin!  That is some nasty looking DOT4 in that bottle!!!  Hope that wasn't your bike.

I just open up the bleeder, put a long piece of tubing on it going to a milk jug half filled with water to keep it from falling over.  Push the tubing into the handle so it won't come out and dump brake fluid all over the place.

With the clear tubing I can see the fluid flow in the tube and adjust the lever return speed to minimize the back flow.  Then I just work the lever until I pump through an appropriate amount of fluid.  Easy peasy and CHEAP!

Thankfully not my fluid I change mine regularly. I was also starting from scratch with no fluid in the system. The valve allowed me to pump as fast as I liked.
Regards Martin.