Duc Monster rear brake issues

[Doc]

Got a friend who sunk WAY too much money into his Duc 695.

Along with a bunch of other mods he put a sssa on it form a Duc s4r.

He is riding along and the rear brake grabs, gradually getting worse and it is smoking when he stops, pretty much locked up. He lets it cool and is able to ride away but it gets hot, starts locking up and he stops again.

He brings the bike to me about a month after the incident rear brake seems to work ok. I replace the rear pads (down to nothing) and bleed the brake. Using a mighty vac I go through a bottle and a half of fluid and just barely get the brake working at full lever pressure.

I take it for a spin and it runs fine, I don't have any brake issues (of course the rear brake isn't bled well and is really not working).

Could this be a caliper piston issue? I am going to rebuild the caliper and see if that fixes it. Any other ideas?

Could it just be I need to bleed the brakes better?

[Kurlon]

If the seals are sticking, the brake will drag, which will cause the fun you've observed.

To bleed that system, you have to take the rear caliper off and hold it up above the master, so the loops in the brake line are no longer the high points in the system.

[Honclfibr]

Well, brake fluid absorbs water. I believe if there's too much water in the fluid you might have the problem described since the water will expand as it heats up creating pressure in the system. And of course the engagement of the brakes causes more heat to be added to the system, which causes more expansion, etc etc...

[Doc]

Well, brake fluid absorbs water. I believe if there's too much water in the fluid you might have the problem described since the water will expand as it heats up creating pressure in the system. And of course the engagement of the brakes causes more heat to be added to the system, which causes more expansion, etc etc...

Would air also cause this?

I will bleed it again as Kurlon suggests and while I am at it I will rebuild/clean the caliper.

[LuvDog]

Since you mention he added a SSSA... did the brake parts also come from that bike? or did he re-use his existing stuff?

[Doc]

They came off that bike.

[Paul_E_D]

Is something putting pressure on the lever? is it adjusted properly? Custom setups like this can sometimes be tricky.

[BostonSVkid]

is the break lever moving freely??

[QuarterGig]

as Honclfibr said

water in the fluid could cause it to. happend to my girlfriends bike and we changed the fluid out and it never happened again.

[Honclfibr]

Would air also cause this?

I will bleed it again as Kurlon suggests and while I am at it I will rebuild/clean the caliper.

No, air would not cause this, air does not have a high coefficient of thermal expansion.

The problem with air is that it's compressible, that's why your brakes are spongy when there's air in the lines.

[Doc]

Is something putting pressure on the lever? is it adjusted properly?

Nope nothing putting pressure on it. Yep adjusted properly

is the break lever moving freely??

Yep.


I am leaning toward old brake fluid with water in it and or a messed up caliper. I'll report back what I find.

[Kid Thunder]

To much fluid in the resevoir.

[Paul_E_D]

Wait! Has something blocked the pinhole in the resevoir cap? That will produce exactly these symptoms...

[Doc]

Wait! Has something blocked the pinhole in the resevoir cap? That will produce exactly these symptoms...

I'll check that.


Another interesting note:

I pump the brake lever and I see the fluid level lower. As soon as I stop the fluid level goes back up.

Is this just the pressure building in the line and when I let go of the lever it releases that pressure?

[Kid Thunder]

You are correct! If there is a blockage in the cap breather or you have to much fluid it will act like you are riding brake.