Front brakes locked

[polkaudio]

A buddy bought a banshee last year that's been serviced thoroughly

by the original owner.

 

His first trip out last season we had trouble unloading it because the front brakes had

locked up. After cracking open the bleeders on both calipers they were fine.

 

Well it seems when it's sitting for a while (only a couple of hours) they will lock up again.

 

The master cylinder and calipers were rebuilt and new brake lines installed

and this problem still persists. This last trip out he just ran with the bleeders open,

so no front brakes.

 

Any ideas of how to cure this will be grestly appreciated. :biggrin:

[odaen]

Is there brake fluid in the lines or something else? I've never heard of that happening.

[Satsol67]

If not that the only thing I can think of is the break line has collapsed. If indeed the master cylinder and calipers were rebuilt right.

[polkaudio]

Yes, there's fluid in the system.

 

They've tried to gravity bleed the system, apparently

Yamaha's can't be done. They've also tried bleeding

the system the same as you would a car. This problem

still persists.

 

The next thing we're going to try is vacuum bleeding.

 

And I doubt the lines collapsed since they're new. But even

so, this was happening last season before any rebuild work

was done on it.

 

I can't see how or why this is happening. It just doesn't make

sense.

[ssanddemon]

The lever is not letting the pushrod (in the master) back all the way. The cylinder cup is not allowing fluid to return to the master reservioir because of this, so fluid which is now trapped in the lines will expand with heat, causing the brakes to apply. Seen it a bunch on cars where a guy installs a new brake master and gets the adjustment wrong, this is a textbook symptom.

[polkaudio]

The lever is not letting the pushrod (in the master) back all the way. The cylinder cup is not allowing fluid to return to the master reservioir because of this, so fluid which is now trapped in the lines will expand with heat, causing the brakes to apply. Seen it a bunch on cars where a guy installs a new brake master and gets the adjustment wrong, this is a textbook symptom.

 

Okay, how do we fix it? Would this be in a service manual?

[ssanddemon]

Probably not. There is no adjustment for the lever, it just pushes against the master cylinder pushrod from a raised section at the back of the lever. This issue can only be caused by a mismatch of parts, like the lever not machined just right or the reservoir feed hole a bit too far forward in the master itself. Something to try: remove the lever and grind back the area that pushes against the pushrod. I bet when the lever comes off, you will find that it has been holding the pushrod back quite a bit from the fully released position. If you had the brakes binding at the time, they would instantly release when the lever comes off. Get a bit of clearance here, and I bet it will then bleed out pretty easy and work OK.