Before embarking on a fault finding mission, I would like to ask the wise forum members if anyone has experienced or know of the problem I am experiencing on my white RS 4V.

The problem is that the front master cylinder piston does not return to it's (its) resting position after applying the front brake. The previous owner already had this problem which the BMW dealer could not sort out. When I got the brick I checked, cleaned the whole system and replaced all the rubbers. But the bleeding was tedious because the piston would not return to it's (its) resting position by the force of the return spring. I needed to improvise a device to pull it back for bleeding. The return spring seemed good. After having blead the system the brakes worked but the piston did not return all the way. Consequently the brake lever was flopping loosely. BTW the brakes were not binding and working ok. Just the lever being loose that I kept forward with a rubber band. 

When I had some time I replaced the piston return spring with a stronger one and that solved the problem ..... until this week when I did a 900km ride. I got a fright when wanting to apply the brakes and there was nothing. Only with the lever right up against the handlebar grip did I have brakes. The brakes were not binding but the piston just moved a bit. 

Where did the brake fluid go? I can't see any leak and am wondering if is disappears in the ABS modulator???

Before I go and bypass the modulator to see if that makes a difference, maybe there is some info out there?

A lot of random brake problems end up being caused by the rubber lines delaminating internally.  A partial delamination can form a flap that works as a crude one way valve.  That's why they're supposed to be replaced every five years and why I replace mine with braided stainless lines as soon as I'm able whenever I get a new-to-me bike.

Good point. I've had that happen on a rear brake line on a Moto Guzzi V1000G5.

I always cut brake lines when I change them. I never saw any delaminated lines. 

But faulty K4V master cylinders, yes! A lot!

There are 2 holes from the brake fluid tank to the master cylinder. One big where you can see the piston moving and another very thin that is necessary to permit the piston to return. It's in the red circle.

When this hole is clogged, the return of the piston is impossible. 
Check it.

Thanks for the response so far.

jbt, when I cleaned the system I did check that the small hole is open, but I am going to check again to make 100% sure. Thanks

If you've got a dead clutch cable, one strand of wire makes a good master cylinder hole poker (??!)

Another option for cleaning the brake master cylinder holes is with a set of spray paint gun cleaning brushes. These are not expensive to buy and come in a set with various sizes. They are also useful for cleaning carburettor jets or any metal items with small diameter holes.

Well, I have checked the holes and both are open. So I'll have to look and think deeper.

Fact is, the brake was working well and the pull on the lever normal, say 1/3 of the way and would be able to lock the front brake. Now the lever is loose, taking up the 2/3 free play and I can pull it right up to the grip for it to brake. 

1. the brake fluid must have gone somewhere. If in the hose, one would be able to see a bulge.
2. the brake is not spongy. Therefore no air in the system and/or a rubber hose expanding and contracting.
3. The brakes are working normally and are not binding.
4. both piston rubbers are new.

Rolf E wrote:4. both piston rubbers are new.

Are you sure they're mounted in the right direction?

Rolf E wrote:1. the brake fluid must have gone somewhere. If in the hose, one would be able to see a bulge.

Not necessarily. I have once - but only once - seen a rubber brake hose deliminate so quickly and so thoroughly that just about all of the hydraulic fluid disappeared into the hose without creating a noticable bulge. That was on a Kawasaki K1100C Spectre back in the 1990s.

My money is still on the master cylinder and specifically, the front o-ring. You don't have an overhaul kit to hand?

I sucked up the brake fluid from the master cylinder and disconnected the brake hose from the master cylinder going to the ABS modulator. Nothing changed. i.e. the piston is still not returning to the resting position.

That means the problem must lie with the master. Tomorrow I'll strip the master and hopefully find the issue.

Thanks for the suggestions. 

I have seen the cup expand due to moisture in the fluid and that prevented the piston returning fully. I considered it was due to a below standard cup being used by the fool that reconditioned the master cylinder.

Wash everything with white spirits and if you have access to a compressor, then blow some compressed air through the master cylinder.  Place a rag over the unit to stop muck going every where.  The usual safety precautions regarding eyes, etc.  Let everything dry before inspecting under a good light and I use dental picks for cleaning the orifices and for any remaining grit found (gently).  Blow through with compressed air again, inspect, then reassemble.

TacKler wrote:The usual safety precautions regarding eyes, etc.

What, you've not mastered the safety squint!?

Two Wheels Better wrote:

TacKler wrote:The usual safety precautions regarding eyes, etc.

What, you've not mastered the safety squint!?

Along with my pair of workshop safety thongs and Stubbies.  

Its hot, it's damned hot in my shed.

My K1100RS had that issue when I purchased it.  It served as a good "hill holder" while riding at stop lights in The Bay Area. I resolved it by removing and relubing the PITA pivot pin for the front brake lever.

My feedback and thanks to all for contributing. It certainly helps as there are so many possibilities. 

Well here is a new one - I am the idiot. What happened is that the adjusting screw had worked itself out over the 200 odd km I had travelled without using the brakes. The piston was in the right resting position but the adjusting screw turned out. I did not notice this when disassembling the unit. Must be an age thing....  The piston had some small burrs from me and others previously trying to get the piston to move. Also where the retaining screw stops the piston. That caused some little scratch marks in the cylinder. I thought that could have caused some binding. The rubbers were still perfect and in the right direction. With a bit of honing the scratches were taken care of and the burrs on the piston dressed. I assembled the unit and then when needing to adjust the free-play ... bingo.... the light came on.  




I must say in my defence that the history with BMW and me originally not being able to get the piston to return until I installed a firmer return spring set me up for the fall.

It was a good lesson. The adjusting screw has no lock nut, so Lock Tight should hopefully eliminate a reoccurrence.