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So I just can't seem to bleed the damn rear system. It’s on my 2000 748. I've used a mighty vac bleed tool, it works @ first, but after 30 minutes or so its flat again, lever goes all the way down with no brake engagement. I've even taken off the caliper and held it up in the air while bleeding and same result. I don't see any evidence of the caliper leaking oil or any other signs of leakage. Anyone else have this issue?
 

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I thought the best technic for bleeding is backwards. Sounds like your a little flusterd, Buy keeping your caliper down low, push fluid up to the resv. instead of pulling from the resv. This is similar to what your thinking. Pushing the air out rather then pulling it out is easier.
 

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Gallep said:
So I just can't seem to bleed the damn rear system. It’s on my 2000 748. I've used a mighty vac bleed tool, it works @ first, but after 30 minutes or so its flat again, lever goes all the way down with no brake engagement. I've even taken off the caliper and held it up in the air while bleeding and same result. I don't see any evidence of the caliper leaking oil or any other signs of leakage. Anyone else have this issue?
Gallep,

If you are getting air in your system in less than 30 minutes, something is wrong!

Before doing anything check your line to make sure it is not damaged. The line should be dry along its entire length. Then check your master cylinder for leaks. Make sure there are no fluid in the rubber boot attached to the master cylinder pushrod. If there is fluid in that boot, the seals on the master cylinder are leaking. Then check the line connections to both the master cylinder and the caliper. If you see any fluid, it is leaking from the threads on either the banjo bolts or the bleeder valve.

Personally, I do not like the Mighty Vac for bleeding. More often or not, using a Mighty Vac actually introduces air into the system during the bleeding. I used to use Mighty Vac to bleed my brakes and was never getting a good result. I eventually was able to narrow it down to air getting sucked in through the threads between the bleeder valve and the banjo bolt. The negative pressure caused by the Mighty Vac wants to suck the fluid. This negative pressure also causes the air to get sucked in through those threads. I now only use the Mighty Vac for priming the master cylinder if the fluid is completely drained in the reservoir.

If all of the above check out fine, I recommend you flush the old fluid and put it new fluid. Before doing that, remove the caliper off the wheel. Remove some of the fluid from the reservoir using a paper towel. Try pushing the caliper pistons in. Go slow and watch the fluid in the reservoir. You do not want to have fluid spill over. Remove fluid from the reservoir as needed, using a paper towel. Pushing the caliper pistons in, will move any old fluid out of the caliper cylinders. It would be best to come up with a way of keeping the pistons all the way pushed in while flushing the old fluid out. I would use a C-clamp on a thin sheet of plastic resting against the pads.

Also remove your bleeder valve, and wrap the threads on the bleeder valve with some plumber Teflon tapes. Teflon will not dissolve when it comes into contact with the brake fluid. This will reduce the possibility of air getting sucked in through those threads. Make sure you are not using too much tape and also make sure you are not blocking any orifices on the bleeder. Once the caliper pistons are pushed in, loosen the bleeder valve and start adding fluid to the reservoir and bleeding the old fluid out. You do not need a Mighty Vac. Just use a clear tubing and a jar. Watch the color of the fluid in the jar. When it starts changing, it means all the old fluid is out.

Close the bleeder valve and remove the C-clamps. Now put the caliper back on the rotor (do not mount it). Keep the bleeder valve pointing upward and bleed the system again until the pads make contact with the rotor. Close the bleeder valve and attach the caliper to the mounting bracket.

This should get all the air out of the system and stop the air from getting into the system through the threads.

If your problem returns, than you have a leak in one of the seals and need to take the master cylinder and the calipers apart to figure out where the leak is.

As all internet service suggestions, you are doing this at our own risk so take all the precautionary measures. If not sure this is within your skills, take your bike to the dealer to have them fix it for you.

-Fariborz
 

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Discussion Starter #5
Best answer I could have gotten, Thanks! I had a feeling I was sucking air through the bleeder valve...
 

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air was getting in through the bleeder threads. After some plumbers tape and pushing the fluid out while replacing with new I got all the air out and its still holding. I'm really happy because I've been riding this thing only using the front brake for 2 years now, and wow I can stop much quicker using both brakes now...go figure! :D
 

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Fariborz, Please copy and paste your write-up in the H.O.Wisdom. It's a great summary! And it's one big reason I don't use my mighty vac!
 
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