Front brake self-pressurizes

[Mr Earl]

Installed new levers on my 2003 M800, all went well.  When I removed the old levers, I noted the position of the adjusters and set the adjusters on the new levers to match.  Since the install, the front brake engagement point is not constant.  We're having hot weather here, and when the bike is parked in the sun for 8 hours, the brake lever has no free play.  So then I've got to back off on the adjuster to loosen it up so I can ride home.  Not an elegant solution.  In the morning in the cool garage, the adjuster needs to be run in a turn or two to give enough engagement.

I bled both calipers last night but today the same thing happened.

Any thoughts?  What did I do wrong here?  The front brake has been very well-behaved until now.

[Howie]

You need 1.5-2mm. at the lever.  If that doesn't work make sure the bypass (return) port is not clogged.

[Slide Panda]

yep - sounds like the return is blocked or nearly blocked.

[Mr Earl]

Thanks.  I've read here about the bypass becoming plugged in other threads, but frankly I've got no idea how to check or fix this.  I'd really appreciate any help you could offer...

[Slide Panda]

Which sort of masters are you running? Gold lines (i'm assuming)

[Ddan]

Thanks.  I've read here about the bypass becoming plugged in other threads, but frankly I've got no idea how to check or fix this.  I'd really appreciate any help you could offer...

Take the cover off the master and gently squeeze the lever.  You should see a little spurt in the reservoir when you pull the lever.  If you don't, the return is either blocked or adjusted wrong.  You may want to lay some towels or something around the master to catch any stray fluid.

[Mr Earl]

Which sort of masters are you running? Gold lines (i'm assuming)

It's got the 4-piston calipers.

Take the cover off the master and gently squeeze the lever.  You should see a little spurt in the reservoir when you pull the lever.  If you don't, the return is either blocked or adjusted wrong.  You may want to lay some towels or something around the master to catch any stray fluid.

When I bled the calipers I left the cover off the master and noticed the spurting liquid when I squeezed the lever.  So this means the bypass is OK...  Hmmm.

I initially suspected air in the system because the contents of the lines/calipers/reservoir expand when they're hot and contract when cold - which brake fluid isn't supposed to do.  Maybe I haven't bled the system properly or enough?

[Ddan]

If it were just air the lever would probably feel spongy and neutral would be hard to get.  Water in the system  is more likely, flush out the old fluid completely and see what that does.

[Slide Panda]

It's got the 4-piston calipers.

Masters - not calipers.

So I might be a little off in my description as this is for masters off a 999, but roll with me. When you replaced the levers, there was some sort of rounded pin/rod that pushes on the piston in the master. On mine that pin is threaded into a cylindrical gizzy with a set screw, and that assembly then rest in the lever. That pin is where you adjust the free play. Turn it too far out, or change it's position further out with different levers and it can block or impeded the return.

It's an easy adjustment to do - just adjust it so you have a bit more free play. Fractional turns can go a long way here, so there's no need to spin it around 6 times or anything like that. Just back it off 1/2 turn and see if that improves things.

*BE AWARE* that you have just changed the point at which your levers pull will start to translate to braking force - moving it further into the levers travel (closer to the bar) so take it easy when you go do any test rides.

Air would really have the opposite effect. Though air will expand more than brake fluid, it also compresses much more readily. Air in the lines leads to a 'spongy' feel at the lever and reduced brake effectiveness

[Mr Earl]

Masters - not calipers.

So I might be a little off in my description as this is for masters off a 999, but roll with me. When you replaced the levers, there was some sort of rounded pin/rod that pushes on the piston in the master. On mine that pin is threaded into a cylindrical gizzy with a set screw, and that assembly then rest in the lever. That pin is where you adjust the free play. Turn it too far out, or change it's position further out with different levers and it can block or impeded the return.

It's an easy adjustment to do - just adjust it so you have a bit more free play. Fractional turns can go a long way here, so there's no need to spin it around 6 times or anything like that. Just back it off 1/2 turn and see if that improves things.

*BE AWARE* that you have just changed the point at which your levers pull will start to translate to braking force - moving it further into the levers travel (closer to the bar) so take it easy when you go do any test rides.

Air would really have the opposite effect. Though air will expand more than brake fluid, it also compresses much more readily. Air in the lines leads to a 'spongy' feel at the lever and reduced brake effectiveness

It looks like I've mis-communicated with some incorrect terminology.  I have coffin masters.  When I disassembled the lever, there was the small pin and the threaded plug (free play adjuster) that I referred to as an adjuster in my earlier post.  When I replaced the brake lever, I set that free-play adjuster with the same amount protruding from the lever as the original had, and the lever engagement felt proper.  Riding the bike over the next few days, the behavior I described began to occur.  So I've actually been performing the adjustment you described, twice a day.  I've been setting the free play when the bike is cool, and again when it's hot.

The fluid quality looks excellent (I flushed and replaced it back in April.)

Any chance that an over-filled master would cause these problems?  (Can the coffin masters be over-filled?)

If it were just air the lever would probably feel spongy and neutral would be hard to get.  Water in the system  is more likely, flush out the old fluid completely and see what that does.

I assume you're thinking it's my clutch master that's the problem?  Although the idea of water in there could be valid...  I have a pretty firm lever at engagement, so it's not air as you all have observed.  But water would behave a lot more like brake fluid, but expand and contract more.

[Ddan]

Try this.  With the free play adjustment on your 'cold' setting, see if you still get the slight spurt in the reservoir when you pull the lever.  I'm betting you don't.

[Mr Earl]

Just tried it.  Squirted up pretty good.  If I pull the lever in really slowly, the fluid will stay still.

[Ddan]

Just tried it.  Squirted up pretty good.  If I pull the lever in really slowly, the fluid will stay still.

If you completely flushed the fluid then I got nuthin'.

[Howie]

Yes, an overfull master can cause the problem.  Brake fluid will expand as temperature rises.  Halfway up the window is good.  A rare, particularly with steel brake lines, but possible cause is a brake line with internal damage.  A tear of the lining material can cause a flap inside the line acting as a one way check valve. 

I'm still thinking insufficient play in the lever. 

[hackers2r]

I was having the same problem after my brake upgrade.  I took out some fluid per some very knowledgeable Ducati techs and all is good.