Hi all, First post to the forum although I have spent countless hours reading through here. I am currently riding a 2012 Gas Gas 280 Raga, all is well other than the rear brakes. The brakes initially work quite well but on a downhill the pedal will slowly drop lower and lower and will eventually bottom out and I will have to let off and take another stab at it. Is brake fade the right term, or should I be calling this something else? Any Ideas? I'm guessing that I probably need to rebuild the master cylinder?

Thanks, Glayne 

Edited March 24, 20215 yr by Glayne
add info

I've only had 50% success with rebuilding master cylinders. If its 9 yrs old I would fit a new one.

 

Water in the fluid can also have this effect.

Good point arthritic, I've had about the same success rate on an 02 TXT, seems like I always want to fix instead of replace, and then often regret and replace rather than fix! I wonder if some seal kits might dry out before they are ever installed?

totty79, I've thought that as well, I'm just surprised that the brake works as well as it does for 3 or 4 seconds before it bottoms out. I will flush tonight, hopefully the fluid I have on hand has less moisture in it than what's in the bike!

Thanks for the replies

Air in system, leak etc usually causes a spongy brake, one thing to check first, if brake is normal for a while before it fades is that the brake isn’t binding while riding. Even a little bit would cause heat, brake fade is when brake works fine then fades as brake fluid boils due to excessive heat. Normally on very heavy prolonged braking but I had a calliper seize causing one side to press on disc and heat it up. Brake then started to fade on use before it seized. Just worth checking wheel spins freely and brake works/releases

Most likely the piston seal of the master cylinder is buggered and oil slowly bypasses as you hold pressure.

I have ridden a trial ABSing the rear brake because I didn't have a rebuild kit.

Good thoughts timdog, pads are wearing evenly and wheel spins freely, break is not spongy and bites quite well but only for about 3 seconds. I changed the fluid yesterday afternoon some came out a little murky but I don't think the break feels any better. Pretty sure kurtas is correct. I think I'll find a rebuild kit, bugger it up, then order a new master cylinder!

Thanks everyone

Start with new pads as people always think they have plenty of pad when its beyond the wear limit. 

While you're poking about, verify that your rotor isn't below it's minimum thickness; if it's too thin it's possible the pistons are over-extending and not applying an even pressure onto the pads.

Thanks lineaway, pads are very new.

Interesting thought d2w, I will check on that.

Moto Trials West shipped me a new master cylinder that I received today, hopefully have it sorted this afternoon.

Thanks all

D2w, I cant even feel a ridge on the disc, bike is fairly new to me but I know it is low use because I know the previous not users! Still on original tires for a few more days. 

New master cylinder, brakes are rock hard. Could have easily lost the spacer on the top bolt and wired up the brake line to keep air from getting in. I'll try to add pics. Easily bled from the master banjo bolt... do they call that a banjo bolt everywhere?

And yes the bike is dirty, has anyone else followed an ambitious 6 year old on a KTM 50? Crazy roost!

kurtas and arthritic nailed it!

Thanks for the help!