Brake Bleeding Issues, Aftermarket MC

smiles79

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So I just got an eBay master cylinder to put on my bike, 1/2" bore. I'm having some issues bleeding it, I get nothing but fluid from the caliper but it doesn't build up enough pressure to light the brake light until almost the end of the handle travel.

I'm assuming I'm trapping air in the softline after the MC (see picture), can this be taken care of by forcing the fluid up from the caliper? I know I should get a proper length hose, but is there anything else I can do to make this work? I imagine it's be fine if I extended the soft hose prior to bleeding but then I wouldn't have a good way of holding onto the MC.

Thanks!
 

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So I just got an eBay master cylinder to put on my bike, 1/2" bore. I'm having some issues bleeding it, I get nothing but fluid from the caliper but it doesn't build up enough pressure to light the brake light until almost the end of the handle travel.

I'm assuming I'm trapping air in the softline after the MC (see picture), can this be taken care of by forcing the fluid up from the caliper? I know I should get a proper length hose, but is there anything else I can do to make this work? I imagine it's be fine if I extended the soft hose prior to bleeding but then I wouldn't have a good way of holding onto the MC.

Thanks!
Yup, you've prolly got air in that loop you made with your long brake line. Try to rig up some way of securing your MC above, like putting it in a vice, or attaching it to a piece of 7/8 stock and having a friend actuate it.
 
You can just take a piece of 7/8" OD tubing and temporarily mount the MC on it in an elevated position. Then vacuum bleed away.

gggGary has posted a method of using the caliper to back bleed the line.

BTW, before proceeding further you might want to replace the line(s) with new Teflon ones of the right length to get rid of the loop. You can find them on e-bay for about 10 bucks each. New Teflon lines offer much better brake feel than old rubber lines.
 
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Alright, I'm getting almost nowhere. I've attached a picture of my current bleeding set up. I've tried the usual method (pump and hold, open bleeder, close bleeder, release), I've tried pushing fluid up from the caliper, sucking fluid down from the reservoir, nothing seems to work. I can pull the handle back to the handlebar and the brake light doesn't come on until the latter part of the cycle. I also started seeing some black smokey looking stuff coming from the master cylinder into the reservoir when I release the handle.

Any other ideas?
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The smokey stuff is likely to be the long term deterioration of your old (old, yes?) hoses. They eventually break up internally.

You might have an air bubble trapped in the junction block.
You could try applying some vibration to your brake lines (run the engine?) to try to break the bubble free.

Or replace both hoses with a single, new hose.
The SS braided lines are not expensive.
 
Yes old hoses are known to collapse internally.
Is the caliper rebuilt both pistons removed seals removed cleaned behind? A common issue is the seals grab the pistons instead of sliding and when you release the lever the pistons are retracted by the seal "stretch" result; the piston just shuffles back and forth, no hydraulic pressure builds.
 
Thanks for the replies. I've been thinking about replacing the brake lines with one line, what is the kind of end that I need to go into the Hardline that runs in between the fork and the wheel and to the caliper? I can't seem to find a hose with an end that will fit.

Also, the caliper is new.
 
Using a single piece line will do away with the "other type" of connector. Both the MS and the Caliper have the same banjo bolt attachment.
 
routing a "full size" line round the fork is tricky? Might have to go round the outside.
 
In my previous reply I meant to say the master cylinder is new, not the caliper.
I mighthave to take another look, because there seemed to be a male fitting going into the caliper, like you'd see on a car brake line. Where do people like to get their stainless brake lines from?
 
A common issue is the seals grab the pistons instead of sliding and when you release the lever the pistons are retracted by the seal "stretch" result; the piston just shuffles back and forth, no hydraulic pressure builds.
Gary,
I've had a variety of brake bleeding issues over the years, but the one you describe isn't a familiar one. Could you elaborate on this piston/seal circumstance, maybe providing a diagram/photo , or a relevant previous posting you've done?
 
If you watch the pads while applying, releasing brake lever you can see the piston go out then spring back instead of going and staying out.
Common cause is not using a bit of brake fluid on the seal/piston during assembly, or not cleaning out the crud that builds up behind the seal, causing it to grab the piston TOO tightly.
Many guys trying to bleed with no success are experiencing this "monkey motion". It's even happened to me!! :yikes: :lmao:
A bit of "spring back" motion is by design, it allows the pads to retract slightly in use so they aren't constantly dragging. But if the piston never slides past the seal at all, the pad never gets all the way to the caliper. Kind of a chicken and egg situation, air in the system reduces piston travel, the reduced travel doesn't break the stiction on the seal, the piston doesn't advance towards the rotor and you can never "build pressure" to bleed out fluid (with air bubbles).
 
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Good luck explanation Gary, I'm starting to wonder if that's what I'm experiencing. When I actuate the brakes, I hear some "creaking" coming from the caliper. Is this the issue you're describing?

I figured it was just a bleeding issue because the brakes worked fine before swapping the master cylinder.
 
I had a similar experience and wound up I had brought it to a shop, they got a nice hard brake back.
 
You may have already done this but I didn't see it. Did you bleed the MC? You should slowly work the lever while watching inside the MC with the lid off. You should see bubbles coming from inside. Sometimes this takes a while and you need to vary your technique to work the bubbles out.
If after all this you still can't get a firm lever I would suspect the ebay MC...
 
Do you mean to do this before connecting lines? Before I installed it I filled the reservoir and cracked the banjo bolt until I got fluid coming bout. I remember initially seeing bubles come up out of the MC when I started bleeding.

The only reason I didn't think the MC was the problem is that it moves fluid and builds pressure, just not until late in the lever travel, which feels like air in the lines.
 
Not before connecting the lines although you could. It's a separate process but it's key to getting a firm lever. Sometimes it helps to tap the MC lightly while you work the lever. You really have to coax the bubbles out. Again, this may not be the issue but I've cured many a poor lever doing this.
 
take the caliper off, and the pads off. pump as much fluid into the caliper as you dare, without popping the piston or seal out. Then crush the piston back into the caliper. that should back-flush the air out of places.

Or let it sit over the winter with the master cylinder as the highest point over the whole system. it'll bleed itself (only works up north.)
 
I'm starting to like the idea of replacing the brake line. I went for a quick ride and the fluid on the master cylinder was very dark. I can stop hard, but the lever will be almost all the way back to the bar.

Can someone chime in and help me figure out what end I need on the bottom of the line if I go one piece? My caliper does not have a banjo fitting, it has a male (flare, maybe) fitting going into it so I planned on keeping the hard line.

Edit: spelling
 
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