Brake Bleeding Assistance Needed

Hi All

After doing my annual installation of new front and rear brake pads, I am having a great deal of difficulty bleeding the air out of the rear brake line.

I am not using the technique described in the topping up the emaster cylinder and then opening the brake bleeder while keeping pressure on the brake lever....which I find takes many many attempts before you get least it has done for me in the past.

How I am doing it (recommended as a superior procedure in a number of articles I read about the issue) is removing the top of the master cylinder (including the diaphram) and then attaching a large syringe (made for this purpose) full of brake fluid to the bleeder, then opening the bleeder and pumping the fluid back UP through the system. The master cylinder fills to the brim and overflows..a lot!!! I then close the bleeder and put the master cylinder back together. I used this approach on the front brake and it worked well.

They say this method is superior because you can move a lot of fluid in one go and it's pushing the air bubbles up through the line...which is the direction all air bubbles naturally want to travel. Well I have put at least 10 ounces (about 350 ml) of fluid thru the system by repeating the procedure 6-7 times but with only minimal results. I have got the caliper piston to finally make contact with the pads put I can't seem to clear the line well enough to actually have properly functioning brakes. However I do have a great deal of brake fluid soaked rags for my trouble.

AM I doing something wrong??

Is there something else I need to be doing with this process?

Do I just need to keep repeating until I get results and accept that it is normal to have to pump so much fluid?

Is the rear brake on the WR450 noted for being difficult to bleed?

I am reluctant to start using the process described in the manual since that means reversing the direction in which I am supposedly moving the bubbles thru the system ...and I have so little brake pressure that vistually nothing will come out of the bleeder.

Any advice, thoughts museings would be much appreciated!


Put a clear plastic tube on the bleeder, and then use the greatest portable bleeder ever made, which is right under your nose.

You can back bleed or slurp fluid instantly.

I had trouble bleeding my rear brake a few weeks ago. The problem with my bike turned out to be a leaking crush washer under the banjo fitting at the MC.

Before i figured out what was going on, I tried to vacuum bleed the rear with no success, then I tried to pressure bleed using the brake lever to supply the pressure. That didn't work so I decided to get creative. I had a spare reservoir cap, so I drilled a hole in it and tapped it for 1/8" pipe threads. The cap was barely thick enough to take the threading, but I was able to get some fittings cobbled together to amke a crude pressure bleeder set up. I think if I had noticed the leaking washer earlier, I could have saved myself some work, but it really wasn't very apparent until I hooked up my homemade pressure bleeder.


It was somewhat cumbersome because I had to stop and remove the modified reservoir cap every time I needed to add more fluid, I'm sure that I could further modify the unit to make it easier to use, but I didn't want to waste time modifying the tool when I was able to use it successfully as is. Here is what it looked like on the bike:


I regulated my compressor down to about 20 -25 psi of line pressure so I didn't blow up the plastic MC reservoir. It worked pretty good, but it was a fairly messy process. The reservoir cap has a couple of small channels that allow atmospheric pressure to act on one side of the cap diaphram. These holes leaked air when I applied air pressure to the reservoir, but it was not enough of a problem to keep the system from working, it just made it kinda messier than it had to be. I saved the set-up, and if I need to use it again in the future, I will make some modifications to make it less messy and cumbersome.

Think I have my problem seems to just have required many many repititions of the process (frustrating, counter-intuitive and very messy) to finally get there. I think I had a fairly big bubble in a high spot and the vast majority of the fluid I was pumping in was just flowing "under" the bubble instead of pushing the bubble out. There seems to be a very limited flow volume that you can pump thru the system in real time so its hard to create a big enough "wave front" of fluid to easily dislodge a bigger bubble.

Thats my theory anyway.

Thanks for the link "rideblue" saving greyracers technique for future reference.

"Zlathim"...I don't have your high level of expertise in things mechanical...but thanks...glad it worked for you.


Yeah, the 'bubble' caught at a high point is a common issue that many fail to realize. If the fluid is not bled quickly enough, the bubble just stays there. Getting the master higher or the caliper lower so the hose is has no high spots does it. Often, if you can get it like that, the bubble will rise on its' own and come out the master port in the reservoir.

Liek William said, sometimes it works better to relocate some of the components to help get the bubbles out of a high spot.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now