07' yz450 rear brake problems

[H 17 Racing]

I have a 2007 yz450 and the rear brake quit working last weekend. The pedal moves full stroke and it dont have any resistance from the master cylinder. If I pump it and try to bleed the brakes it just dribbles out. I tipped it on the left side and lifted the rear of the bike and still no change. The fluid is full and I refilled it when trying to bleed the brakes.

Does anyone have any ideas? Any help would be greatly appriciated.

[2grimjim]

When you were trying to bleed the brakes was the fluid level in the master cylinder dropping?

If the fluid was being sucked from the master cylinder and nothing coming out of the bleeder screw you have an airlock that needs to pass.

If there are no visible leaks from the master cylinder and caliper (aside from the mess from the bleeder screw) you just need to continue with bleeding.

If the rubber seal under the master cylinder cap (where you pour fluid in) had any air trapped above the fluid it may have caused the problem.

When you done bleeding, refill the master cylinder and make sure it is full to the top before putting the cap on. Let the rubber seal under the cap force out the excess fluid before putting in the screws.

[H 17 Racing]

I havent seen any air come out of the bleed screw, and I drained it for awhile. I pulled resivor cap and filled fluid to top and pushed the brake lever, the fluid leved didnt move at all.

[2grimjim]

Ok. Just leaving the screw open isn't going to bleed the brakes. Here's the procedure for bleeding:

1) Take the cap off the master cylinder.

2) Fill the master cylinder TOO THE TOP with brake fluid.

3) SLOWLY pump the brake pedal five or six times.

4) HOLD the brake pedal all the way down with your right hand.

5) Using your left hand, use an 8mm box end wrench open the bleeder screw 1/4 turn WITHOUT LETTING UP UN THE BRAKE PEDAL.

6) Leave the bleader screw open for 5 seconds and close it. DO NOT LET UP ON THE BRAKE PEDAL UNTIL THE BLEEDER SCREW IS CLOSED!

7) Repeat steps 1 through 6 until the brake pedal feels normal again. Be sure to not let the master cylinder go dry........keep it full!

[H 17 Racing]

That is how I was doing it. Sorry I wasnt more specific.

[2grimjim]

It might take repeating the pump/bleed cycle 10-20 times before all of the air is expelled........but, If you aren't getting any fluid movement then you may have a bad seal, or corrosion inside the master cylinder.

Sometimes bleeding brakes takes a lot of patience and persistance.

[grayracer513]

The rear brake has a nifty built in place to lock a bubble up right at the banjo fitting where the hose attaches to the master cylinder. Fluid will flow right under it and never move it. Do this:

> Loosen the line at the master cylinder. Push the caliper piston back a good distance by prying on the pad.

> With the fitting still loose, push the pedal down an inch or so, and HOLD IT there.

> Tighten the fitting, add fluid to the reservoir, and release the pedal.

After that, any further air that sits downstream should be easier to remove. Remember that you will need to pump the pads back down. Watch the fluid level.

[H 17 Racing]

I bleed the brakes backwards, and thier is no air in the line. After doing this I pushed the brake pedal and it moved the piston out about a 1/8 inch and it quit moving. When I look through the glass on the side of the master cylinder I can see air bubbles go to the top every time the pedal is pushed. Does that mean the master cylinder needs replaced.

[grayracer513]

It means there is air in the M/C bore, and it is being pushed up into the reservoir through the fill port as the piston begins to move. How it got there is the question. If you haven't tried bleeding the air from the hose fitting as I suggested earlier, it's worth doing.

But one thing about this situation is what you said earlier: the brakes were working and then quit suddenly with no intervention or visible damage. If air keeps getting back in with no apparent leaks, you could very well have a master cylinder in need of repair. Kits are available, and worth a try for the few dollars they cost.

[William1]

One other possibility is the brake lever is pressing on the M/C piston all the time, causing it to be up in the bore, covering the bleed/fill port. As the pads wear, the system is unable to replenish the fluid, thereby causing the brake pedal to require more and more travel. It explains the dribbling too.

[H 17 Racing]

I got them working this afternoon. I moved the brake lever down and bleed them backwards, opened the banjo bolt on the top of the master cylinder, then pushed the brake lever up and down slowley. Then the air bubble came out. Thank you for all your help