front brake wont bleed


4 replies to this topic
  • tammie

Posted April 25, 2007 - 09:56 AM

#1

I know this is the YZF forum but you guess might be able to help here....

ok so I wanted to overhaul the front brake and caliper on my sisters 2001 trr125 it was having some problems seizing last year so we went and bought new piston seals, and pads. popped the pistons out with compressed air, clean everything really well took the line off and the master cyclinder...blew compressed air through the line to get all the old fluid out and any debris that may have been in the line. Also i blew a little air in the master cyclinder to clean it out, that little rubber valve inside the master resevoir flew out and i don't know which way it supose to go back in we put everyting back together and i can't get it to bleed, im not seeing any air bubbles coming out we tried the rubber valve both ways, there was alittle bit of air bubbles but not like its supose to we've been working on it for 30mins and still can't get it to bleed....

to top it all off my one man vacumn bleeder isn't working either thought that could be of help but of course things break when you need them.

i hope the air didn't do anything is it bad to blow some air into the master cyclinder i was just trying to clean out the old oil and debris.......can anyone help me

  • grayracer513

Posted April 25, 2007 - 12:36 PM

#2

Look at the check valve assembly in the bottom of the master cylinder reservoir. If it has a screen as part of the assembly, and that screen is still attached to it, the screen goes on top. If not, the valve needs to be installed so that it has the least resistance to fluid flowing out of the reservoir, and the most resistance to fluid flowing back up. It's not a one-way-only valve, just a one-way metering valve. Once it's back where it belongs:

> Fill the master cylinder reservoir. Remove the brake line from the cylinder and hold a finger over the outlet port. Now pull the lever just far enough to move the master cylinder plunger 5mm or less. You should see bubbles rise from the valve. Keep doing this until the majority of the air is out of the cylinder bore. Fill the cylinder to the top, set the cap on it, and install the top of the brake line.

> Open the bleeder screw. Remove the cap. If you see no evidence that fluid is draining out of the reservoir into the cylinder (the level isn't getting lower), bump out a couple of bubbles like you did above and see if you can get it to start flowing. If it doesn't, close the bleeder, and patiently tickle out as much air as you can. If you stop getting bubbles to come up from the fill port, try opening the line and see if the fluid will drain through. If not,

> attach a clear hose to the bleeder and apply some vacuum to it to pull fluid down over the air pocket in the line. See if it will flow on its own by gravity.

>once the fluid starts flowing through on its own, which it eventually will, it's just a matter of keeping the reservoir full until you don't see any more air coming out of the bleeder, and clear, clean fluid is all you see. Don't let it run dry during the draining process. Close the bleeder, fill the reservoir, cap it up and test it.

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  • tammie

Posted April 25, 2007 - 02:21 PM

#3

thanx gray, only i was doing this, got my vacumn to work....eventually we were getting fluid down to the bleeder on the caliper, and bubbles, but not once were bubbles come out the top in the master cyclinder resevoir(whenever i change brake fluid or put a new line on my yz400 when bleeding i get tons of bubbles coming out at the master cylinder while im bleeding) this TTR wasn't having any bubles coming out the mastercylinder....but regardless fluid was going down the line as it was cmoing out the bleeder.....

now i couldn't get any resistance we tried for a long time pumping the lever, holding lever in, using vacumn suction at that bleeder, closing bleeder, pumping lever etc etc etc.....could not get any resistance???

finally gave up.....shouldn't bubbles be cmoing out of the mastercylinder at the top?

  • tammie

Posted April 25, 2007 - 02:22 PM

#4

oh i just re read i will try removing the line at the master cylinder and holding my thumb over it and seeing if bubbles come out of that valve......

  • grayracer513

Posted April 25, 2007 - 03:47 PM

#5

What will sometimes happen is that a bubble will get stuck at the top in an area large enough to allow the bubble to stay where it is and let fluid flow around it. Under those circumstances, you can bleed the brakes all day and get nowhere.

When the master cylinder piston sits at rest, the piston seal is back behind the fill port, which allows fluid to enter the MC as the brake pads wear, and self adjust. When it travels that first 2-5mm, the fluid directly ahead of the piston is forced up out of the cylinder and into the reservoir until the port is closed, and fluid is forced to exert pressure on the caliper piston(s). This is what you are trying to take advantage of by the "tickling" technique of only moving the lever that small amount. If there is air at the top of the system, you can get it to pop up and out through the fill port.

But air wil always go to the high spot, too, and you may have to turn the bars to the left so that the cylinder tilts in that direction and encourages whatever air there might be to slip up to the fill port.





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