R90/6 brake bleeding question

redavide
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Joined: Tue Dec 04, 2007 12:33 pm

R90/6 brake bleeding question

Post by redavide »

Just completely rebuilt the master cylinder and calipers on an R90/6 with dual front discs (like R90S).

Now I'm trying to bleed them, but having some trouble. I just filled the MC with fluid and started to pump the brake lever with one bleeding nipple open. Doesn't seem to be working. Do I have to open both nipples? Should I get a syringe and push the fluid up from the caliper into the MC?

Any suggestions on best method(s) after a rebuild?

Thanks.

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schrader7032
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Location: San Antonio, TX

Re: R90/6 brake bleeding question

Post by schrader7032 »

So I take it you have one hose to one caliper and then a "jumper" hose over to the other caliper? Never been in that situation, but like bleeding brakes on a car, the way is to bleed the brake that is farthest from the source. So, if you have the jumper situation, I'd say you should open just that nipple and work that one until fresh fluid comes out. Then go to the other one and repeat.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

redavide
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Re: R90/6 brake bleeding question

Post by redavide »

Kurt, No, there is no "jumper." I have one tube connected to one caliper. The other caliper's bleeding nipple is closed.

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schrader7032
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Re: R90/6 brake bleeding question

Post by schrader7032 »

So, then you just bleed one at a time, right? The master cylinder will push fluid to which ever side has the nipple open. It may take quite a few pumps to begin to get a firm lever. Since you have rebuilt the master cylinder, I take it that means there's no rust or roughness in the bore. If that's the case, then you can pull the brake lever as much as you want. When I bleed my brakes after some time, I use a piece of wood or something as a limiter on the lever so I only move the lever about as much as I do under normal braking situations...that way I don't pull the master cylinder seal across any areas that might have degraded over time.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

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Randolph
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Re: R90/6 brake bleeding question

Post by Randolph »

I fabricated this Air Cadet vacuum pump (Cole Parmer) setup for bleeding the brakes in my Suburban for a one man show. It pulls a lot of fluid through so keep an eye on the M/C level. I know how many seconds it takes to deplete the M/C in the Suburban. Just turn on the pump and then open the bleeder valve until the M/C level gets low (don't let it hit bottom) then close the valve and turn off the pump. Refill the M/C and repeat. Use a thick wall Nalgene catch bottle.

Since the bleeder valves are a straight thread there is air that comes thru as bubbles in the vacuum line. Not to worry, the air can't get into the caliper with vacuum pulling it through.

I have an R90/6 w/dual ATE calipers, splitter under tank and 13mm handlebar M/C.

I flush the fluid through both vehicles every year using this setup.

Randy
Attachments
Double hole stopper
Double hole stopper
Cadet brake bleeder
Cadet brake bleeder

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srankin
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Location: Spencerport, NY USA

Re: R90/6 brake bleeding question

Post by srankin »

Kurt gave good advice.
OKAY, did you change the pads? On the ATE calipers there is a cam system which needs to be adjusted to properly align the pads to the rotor. A mis alignment of the pads will give a poor brake feel. If you didn't change the pads I would still give them a check for alignment and wear, they don't wear evenly.

To bleed, pump the lever a few times then hold it with pressure on it and quickly pop open the bleeder nipple, you should get a clear stream of fluid once the air is all out. Don't keep the nipple open too long and don't pump the lever while it is open or you could get air back into the system. As mentioned a vacuum system works darn well if you have one. Unlike the rear brake on my RS, there is no spot I can think of that would trap air.

Now, last thing, and I just found this on a fellow's bike I was helping him with, is the cable from the handlebar lever to the under tank master cylinder. On my friend's bike we knew we had all the air out of the system and the brake pads properly aligned still the lever went to the handle bar. Cables stretch over time and all we did was adjust the brake cable and that solved our problem. So, check for a worn out of adjustment brake cable.

Hope this helps good luck, St.
Owner of a 84, R80RT and 78, R100RS

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Randolph
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Re: R90/6 brake bleeding question

Post by Randolph »

I offer the vacuum bleeder method as a REALLY quick way to get the air out of the system after it's been opened up as you have done to yours.

There are places for air bubbles to hide in any brake system. Tom Cutter (http://rubberchickenracinggarage.com) has mentioned that an air bubble can be trapped at the front of the M/C bore. He has had to tip the M/C vertically and used a syringe to expel the bubble. It sounds messy, but it worked.

I believe there are off-the-shelf vacuum bleeders like the Air Zapper Brake Bleeder that Brook Reams uses.

Randy
Last edited by Randolph on Wed Jul 21, 2021 7:59 am, edited 1 time in total.

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srankin
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Re: R90/6 brake bleeding question

Post by srankin »

I do use a vacuum system I rigged up with an electric pump and catch bottle. It is by far the best way to bleed brakes as well as suck out the fluid in the MC when changing fluid.

Still, like I said, there are other things on the system which can cause a wonky brake feel. BMW made the right choice in going to the handlebar MC and Brembos. Unfortunately, unless we spend money to covert or redesign the system we have, we are stuck with it and the pain of maintaining it. Not that the Brembo system is without it problems bleeding as well. Try bleeding the rear brakes on a 78 RS. St.
Owner of a 84, R80RT and 78, R100RS

Daves79x
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Location: Knox, PA. USA

Re: R90/6 brake bleeding question

Post by Daves79x »

Teflon tape all the bleeder threads, especially when using a Mity-Vac or other vacuum bleeder. No renegade air bubbles that way.

Dave
Dave

chrishea
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Re: R90/6 brake bleeding question

Post by chrishea »

Let’s talk technique here: “I just filled the MC with fluid and started to pump the brake lever with one bleeding nipple open. Doesn't seem to be working.”

The bleed nipple needs to be closed when squeezing the brake lever, then opened, the bleed nipple then is closed before releasing the brake lever, otherwise air just gets sucked back in. So: squeeze lever, open nipple, lever goes to the bar and hold, close nipple, release lever. You might need a helper. Then with the nipple closed, pump the lever a few times, add fluid if appropriate and then repeat the cycle until no more air comes out. With two calipers you’ll need to do that on both sides.

The vacuum techniques work well too, I use a vacuum set up, but after the vacuum bleed I finish off with one or two final strokes as discussed above.
Chris
Seattle

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