newbie front brake problem

slparry · Post by **slparry** » Sun Apr 28, 2013 8:24 pm

So as I understand your problem

1. The brake works perfectly when parked?
2. The brake works perfectly when used normally/moderately?
3. The brake goes mushy when used hard?

Although you've replaced the master cylinder it could be possible that it's piston seals where dried out and damaged, and that your old one could also have failed master cylinder piston?

To me, the mushiness would be present all the time if the problem were air in the system.

I suspect there's moisture in the system and as you use it harder the moisture turns to vapour causing the mushiness.

Are you sure there's no leaks anywhere? What type of brake fluid are you using?

Boxered · Post by **Boxered** » Sun Apr 28, 2013 8:26 pm

Could this be a caliper problem rather than MC?

Steve

Dai wiskers · Post by **Dai wiskers** » Sun Apr 28, 2013 10:34 pm

Another idea are the callipers binding when hot causing the fluid to overheat in the callipers
It now sounds like a sticky piston or two
Do the callipers feel hot after riding if you have a centre stand are the pads binding on the disc's

pauls · Post by **pauls** » Tue Apr 30, 2013 6:54 pm

pads bind slightly but no more than any other bike I have had? still doesn't explain going in garage and having no pressure? cheers

slparry · Post by **slparry** » Tue Apr 30, 2013 7:21 pm

pauls wrote:pads bind slightly but no more than any other bike I have had? still doesn't explain going in garage and having no pressure? cheers

I think you may have a duff master cylinder seal, just because you fitted a used master cylinder doesn't mean the replacement wasn't also damaged.

I'd look at getting a master cylinder repair kit

http://www.realoem.com/bmw/showparts.do ... g=32&fg=72

Something, somewhere in the back of my mind I have a recollection of some BMW master cylinder having an issue where the seals could be damaged if the lever was squeezed too far when the bleed nipple was open or something. I could be wrong but it's floating around on the edge of my befuddled brain

r550s · Post by **r550s** » Tue Apr 30, 2013 8:17 pm

Only took me six years to crack this problem...
Maybe the air bleed on the reservoir cap is blocked - two of the screw holes in the cap have a small slot in them. The purpose is to allow air into the bellows - allows the bellows to expand as you wear the pads - on your hell for leather long run. If the slots are blocked by copaslip (yep, that's me), then you'll get your exact symptoms. Leave the bike overnight and some air creeps in, or, remove the cap to have a nosey and - magically - the problem disappears.
My epiphany arrived in a layby, just leaving Oban two years ago. I rode three hundred odd miles home with a smile on my face all the way - smiling at my own dumb stupidity.

slparry · Post by **slparry** » Tue Apr 30, 2013 8:28 pm

r550s wrote:Only took me six years to crack this problem...
Maybe the air bleed on the reservoir cap is blocked - two of the screw holes in the cap have a small slot in them. The purpose is to allow air into the bellows - allows the bellows to expand as you wear the pads - on your hell for leather long run. If the slots are blocked by copaslip (yep, that's me), then you'll get your exact symptoms. Leave the bike overnight and some air creeps in, or, remove the cap to have a nosey and - magically - the problem disappears.
My epiphany arrived in a layby, just leaving Oban two years ago. I rode three hundred odd miles home with a smile on my face all the way - smiling at my own dumb stupidity.

Cool

pauls · Post by **pauls** » Tue Apr 30, 2013 8:51 pm

ok cheers will look cheers

Corvus · Post by **Corvus** » Tue Apr 30, 2013 9:23 pm

The blocked reservoir vents are food for thought. The system is ultimately atmospheric.

Do we yet know if this machine is abs?

Do we fully know the symptoms? A hard run or "the twisties" brings on the symptoms, but are we to presume that means heavy use of the brakes or just any use of the brakes? What happens if the brakes are given a man size squeeze after a very fast run on a straight road? Is the problem associated with heat or just repeated use of the lever?

Are we using the recommended grade of fluid?

Is the problem definitely hydraulic? Aside from possible abs problems, are the pads located correctly? Are they the right pads (sorry, got to ask)? Is the pad friction material still bonded to the metal backing? Are the pads oil contaminated (sorry, got to ask)?

If the floating bushes are worn and the disc is also worn, excessive disc movement may possibly push the pads back. The pronounced rim around the edge of a worn disc may move around if the floating bushes on the disc are worn. Long shot, but we're starting to need long shots!

DaveH · Post by **DaveH** » Wed May 01, 2013 8:14 am

Have you run the bike over winter or left it standing.

Had a Triumph Sprint in this week with exactly the same problem.

Check the edges of the brake pads to make sure they are not scabbed with rust.

Dress lightly with a fine file if they are.

The rust prevents free movement of the pad in the calliper and can result in the pads not 'following' the piston exactly.

It only takes a fraction of pad to piston clearance to mean that when you pull the brake on, the pressure is used taking up all the slack and there's nothing left for braking effort.

Dai wiskers · Post by **Dai wiskers** » Wed May 01, 2013 8:31 am

I'm liking Corvus more and more

Another thing I would try is (presuming you fitted double lines from the master cylinder ) running with just the one calliper connected using the banjo from the unused calliper at the master cylinder be careful to seal the unused calliper well
Repeat with the other calliper this should enable you to find out if it's only one side playing up or both if both the problem must be with the master cylinder if just one side it must be that calliper
Be careful doing this as your insurance won't pay out if you dump it

I do like the cap screw idea

Another thought you did strip the master cylinder you fitted
What lubricant did you use when you rebuilt it?
Red grease good
Brake fluid good
Lithium grease bad
Copper grease bad

pauls · Post by **pauls** » Wed May 01, 2013 2:01 pm

its not abs and its only done 18k discs fine pads new. ok that's great gives me something to go on, off on a short break now back mon so will have a go then cheers to everybody,

pauls · Post by **pauls** » Wed May 01, 2013 2:26 pm

damn quickly took off cap slots are free, so to recap there is a set of bends round the corner from my house, I can ride bike all day at sensible speed and brake is spot on, I then go down to my favourite bends at speed a 800 metre long straight hard on brakes sharp right brake spot on 600metre straight sharp left brake ok 800metre long straight very sharp right brake comes back to bars? go along bout 800metres sharp left no brake 1000metres more and a steady left brake spot on again? I then go home put bike in garage and say 2 in ten times I go to get bike out again and no brake? cheers

simon · Post by **simon** » Wed May 01, 2013 4:59 pm

... master cylinder seals?

conkerman · Post by **conkerman** » Wed May 01, 2013 5:13 pm

Backfill the brake system using a big syringe.

If that does not work you need to be having a close look at the calipers/master cylinder and ensure they are not leaking.

Loss of 'pedal' can only really be from air in system or a duff seal.

Did you bleed the brakes whilst on the sidestand??