brake bleed help 88RT

[9842]

Hi
Looked around and could not find info on bleeding brakes so thought I would ask for help.

Installed new master cylinder on 88 r100RT. Filled reservoir and squeeze lever, loosen bleeder, tighten bleeder, and release lever, 50 times. Used a sucker and cracked bleeder, sucked out 4 or 5 reservoirs on master cylinder. es I filled before sucking air. Still no pressure so back to squeeze, open, close, release. Still no pressure.

I quit and tied lever pulled back to bar and will leave it overnight.

Any magic words appreciated.
Thanks
Brian

 

[pauls1150]

The "proper" new master cylinder, and not an el-cheapo knock-off...?
Lots of pipes and unions... maybe an old copper crush washer?
Stock rubber lines? It's time to change them anyway, preferably to braided steel lines (and you'll love the improved "feel" since they don't expand when the brake is applied).
Do you have a speed bleeder on the caliper? Maybe it's not sealing closed completely, or the threads seep; I seem to recall that some of these were too long to seat properly.

 

[GTRider]

Most likely there is air trapped in the master cylinder bore. Some master cylinders (usually on newer models) have a bleed screw at the MC for removing air trapped in the bore. On older MC it often helps to crack loose the brake line to MC fitting and carefully bleed there, using the same crack-compress-tighten technique—after carefully protecting all the nearby surfaces. Followed, of course, by a thorough soap and water washdown.

Best,
DeVern

 

[88bmwjeff]

I've found it kind of hard to get rid of all the air bubbles in the U-shaped pipe that goes from the left caliper to the right caliper. It's kind of best to bleed the brakes with calipers loosely attached. You can unbolt the calipers and move the assembly around to get rid of the air bubbles out of the U-shaped portion. I can't say that's what's going on with you, but it's a possibility.

 

[pauls1150]

Following up on DeVern's notes - Question: If it's actually the master cylinder, is there a possibility that the "new" piston seal is not is just not properly sealing in the "new" bore?

 

[AntonLargiader]

I would start by turning the bars fully to the left (so the right handlebar is pointing up to the right) and squeeze the lever a few times. That will work out any air that's trapped in the cylinder. If that doesn't do the trick, remove the calipers and spread the pads to force fluid (and any air bubbles) back up into the MC, and then try the first step again.

If there was no air in the hoses and calipers before, just for replacing the MC I'd avoid bleeding the fluid downward until the air is already gone.

 

[ccolwell]

I quit and tied lever pulled back to bar and will leave it overnight.

This often works. I wish I knew why.

And ditto on cracking the brake line to MC.

 

[pauls1150]

Why: the constant pressure encourages the little air bubbles to migrate back up to the top.
But if there is NO pressure, as in having the lever all the back to the bars, it may not work.

 

[9842]

Hi
Got news.

First, I have been bleeding brakes off and on every two years, or so. Noticed front wheel binding a little and after I cracked the bleeder the pressure was released. This went on for a few years. So I changed both lines with spegler SS lines a few years ago. Thought I solved draging brake but not so.

Next step was new master. Ordered a #13 master cylinder from Rusty at Max along with dot 4. Yesterday I installed MC and was not getting the bleed pressure I expected. I tied lever against bar overnight and this morning nothing. Lever just flopped back and forth. I thought, take it apart and look at it because something is not right. After removing MC I turned bars so I could look into where the MC plunger fit into the throttle assembly and "bingo". I left the old rubber gromit in place. After removing the old gromit I reinstalled the new MC, gromit, and washer, and I had pressure, yeahhhh!!!

Finishing up and doing a final bleed. Feeling like a dunce but glad it's fixed. I thank you guys for chiming in and helping me out.

Thanks again
Brian

 

[ccolwell]

Why: the constant pressure encourages the little air bubbles to migrate back up to the top.
But if there is NO pressure, as in having the lever all the back to the bars, it may not work.

That's been my guess as well. Just how the fluid dynamics of air bubbles in brake works like that remains a mystery to me.