Front Brake Blueprinting

[twh_mn]

I am trying to find information on a company featured in an arcticle in either Motocross Action, Dirt Bike or Dirt Rider in the last 2 years. The arcticle was about a company that would blueprint your brake system, ie clean, inspect, check line pressure to specs etc. I am currently chasing an issue with my front brakes. Rather than throw additional money at it, I replaced the master cylinder and brake line last season after a master cylinder rebuild kit didn't alleviate the problem, I would like to have the system performance measured. I am relatively confident that I am properly bleeding the system (Mighty-Vac). Chasing this issue around is interfering with my limited riding time. Any help would be appreciated.

Thanks

[ September 14, 2002, 05:04 PM: Message edited by: twh_mn ]

[michaeltrundle]

Did you still have the same problem after replacing the line and master cyclinder? If you do, then I would suspect the problem is most likely to do with the caliper.(nothing else left)

Why dont you try and borrow another caliper from a friend and see if you can get the brakes to pump up properly with it in place.

This might give you another hint as to whats causing the problem.

The seal around your piston in the caliper grabs a hold of the piston fairly tightly to seal the fluid in. As well as this, as you put the brakes on the seal lip stretches a little outwards towards the rotor with the piston, when you relese the brakes the seal retracts back to its normal position pulling the piston back with it away from the rotor, providing some clearance between the pads and rotor so your wheel can spin freely again.(I'm talking very minimal movement/clearance here)

As your pads wear, the piston is supposed to gradually slide further and further out of the seal to take up the freeplay between the worn pad and the rotor, the fluid in your master cylinder resevoir then tops up the line when you release the lever and you should always get the same amount of lever travel no matter how much your pads wear. (in theory)

The problem I believe is caused by this.

As your piston moves out of the seal the part that is closest to the back of the pad and exposed to whatever coditions you are riding through, starts to get dirty and sometimes corroded. The next time you replace the pads you have to push the piston back into the caliper and seal. Then when you try to pump up the brakes the seal can get caught on the dirty or corroded surface of the piston and it wont slide out to take up the clearance as your new pads wear in. The seal holds onto the piston too tightly, so to speak and wont let it slide.

As you pump and pump and pump during your bleeding process, the piston has to move too far out before it pushes on the pads and then it pulls back as you release the lever etc. etc. etc.

You have too much clearance between the pads and the rotor which requires a lot of lever travel before the pads contact the rotor and things start to firm up, usually just before the lever touches the bars.

The magic 'zip tie around your lever and bars' trick was invented to allow the seal to creep back around the piston after a while. Thats why you leave it on there o/night.

You may just have to dissassemble the caliper and clean everything up properly so the piston can slide out of the seal to provide the correct clearnce between the pad and rotor as your pads wear.

Older kx's used to have this problem and I spent many hours in the shed pumping and bleeding and swearing at the freakin things before I found out what causes it.

Clear as mud eh'

hope this helps and doesn't confuse you, I do jibber on a bit.

This only stands true if you are sure you have all the air out and if you are using a vacuum bleeder then I presume you do.

mick