Air getting into brakes

**mattc** · 17th January 2011, 19:47

Hey guys.

I have a '06 ZX6R as a trackday bike.
I seem to have trouble with air getting into the front brakes as i'm sure that i'm bleeding them sufficiently.

After a few laps however i start to get progressively more lever travel and upon checking at the end of the session i can get a decent amount of big air bubbles from each caliper. After bleeding so i get no more bubbles the same thing happens the next session.

I have noticed a few times that brake fluid has seemingly sprayed onto the inside of the screen - if fluid can get out then air must be able to get in right? Would air getting into the reservoir be compressed and sent down to the calipers or would it just sit on top of the fluid?

There are no fluid leaks i can see anywhere else and i have tried different calipers so dont think it would be a caliper issue, piston seals etc.

Any help appreciated as i'm getting sick of varied lever travel/feel!

**pete376403** · 17th January 2011, 20:00

Moisture in the fluid boiling when the calipers heat up?

**mattc** · 18th January 2011, 20:54

Originally Posted by pete376403

Moisture in the fluid boiling when the calipers heat up?

I wouldn't have thought so as it is new fluid, the motul racing stuff.

**Motu** · 18th January 2011, 21:09

Sucking air into the master cyl on the return stroke.

**Max Preload** · 18th January 2011, 23:40

Originally Posted by Motu

Sucking air into the master cyl on the return stroke.

What he said.

**mattc** · 19th January 2011, 05:50

Originally Posted by Max Preload

What he said.

Is the fix just to rekit the master cylinder?

**2 wheeled worx** · 21st January 2011, 21:21

Your master cylinder will need a rebuild you can buy seal kits to do this and very easy to do remover lever and plunger cover boot and will be a circlip remove the pull plunger out i use a toothbrush with brake fluid to clean any old debris out use brake fluid on new seals when refitting maby try replacing top sealing washers where lines bolt to master cylinder to just in case

**mattc** · 22nd January 2011, 08:01

Originally Posted by 2 wheeled worx

Your master cylinder will need a rebuild you can buy seal kits to do this and very easy to do remover lever and plunger cover boot and will be a circlip remove the pull plunger out i use a toothbrush with brake fluid to clean any old debris out use brake fluid on new seals when refitting maby try replacing top sealing washers where lines bolt to master cylinder to just in case

Cheers for the info, kit ordered!