front brake bleeding probs

09 KTM 250 XC. Just rebuilt front brake caliper after cleaning- new orings in pistons bores, new rubber boots, new pad seats, new retention pin for pads. Rebuilt master cylinder-new piston and seals. new core moto brake line and banjo bolts. can't get system to bleed. no pressure when I pump lever.  Any help would be appreciated. 

have you tried back - bleeding ?


Could it be a scratched piston in the hand control, maybe a seal. Did you check the whole brake system or just down in the wheel?

soanz- did not back bleed but did use mighty vac. 

killabyte- replaced the piston and seals in the mastercylinder also. 

So do you have fluid all the way through?  (ie--from the top, master------>the bottom, caliper bleed screw)

My experience is that it can be extremely hard to get fluid moving through a "dry" that has just undergone a major operation by which almost all of the fluid has been drained.  Pre-fill each component with fluid, using whatever tubing or syringes you have.  Caliper, hose, master.

I have often resorted to removing the system from the bike and optimizing the positions of the master and caliper such that there are NO bends whatsoever in the line and everything is running "downhill" to the caliper.  I have also used pressure applied atop the master to help push the fluid.  Or a "gravity" bleed

When I have issues with front brake bleeding I use the quick and dirty "knee to push the piston in" trick. Hold the wheel at 9 & 3, knee on the caliper, push to bottom out the piston. Make sure the reservoir is full, pump the brake lever. Rinse, lather and repeat until you have good pressure. It helps to have an observer to note whether or not you are getting bubbles in the reservoir. 
As always, YMMV...

tried the knee to caliper trick. seems very stiff-not much give. used all balls rebuild kit for master cylinder piston. doesnt seem to pump much. got a syringe to try to back bleed and see if that works. pistons were very hard to get back in- should they slide in easily?

They are an absolute twat to bleed. Another suggestion is to remove the lever and use a drift to pump the piston as you get a longer stroke. Once you feel you are getting there, replace the lever and pull it in as far as you can and zip tie it to the bars overnight. Works for us. Bob.

thanks. i'll try that



YES ! Got it. put Teflon tape on bleeder screw to stop air leakage. reverse bleed with syringe and it worked like a charm. brake feels better than ever. test drive on fri after sleeping off the turkey and hopefully good to go. no luck with the mighty vac but not good w that anyway. Thanks for all the help on TT. saved me a trip to the shop. 

For future reference, just wondering if you can elaborate on your process with the syringe, and how this is different than with a mighty vac.   The mighty vac pulls a vacuum at the bleed screw, which pulls out any air in the caliper until you start flowing clear fluid, right?   I'm guessing you are using a syringe at the bleed screw to do basically the same thing?  Start with the syringe depressed, and then pull the plunger to pull fluid and air out?   Are you using a short piece of clear hose/tubing between the syringe and the bleed screw?

I think he's back bleeding

sorry for the confusion brent72. out of town for a few days. Yes , I did back / reverse bleed .  I used a 30cc medical irrigating syringe- 60 cc would be better. I had a box of these in my office that I use a lot ( dentist). they have a luer lock on the end to screw a needle onto. this worked great to screw the bleeder hose onto for a tight fit. first drained most but not all the fluid out of the master cylinder- it was already clean so I did not have to worry about changing out bad fluid. back filled syringe with hose on end with clean fluid. go slow to trap less air in syringe. you will still get some air in.  invert hose so it is pointing up and shake/thump to get air bubble out of syringe. depress plunger a little to get  all air out.  Then hook tube onto bleeder valve and open valve. push plunger on syringe to slowly push fluid up the line into master cylinder. be careful not to overflow master cylinder. close bleeder screw and done. I thought I had all the air out of the system with previous attempts. I was surprised how much was still in there. . worked great. has good brake pressure right away. tested this weekend and they worked great. I also removed the bleeder screw before I started and put some Teflon tape on it to prevent any air from leaking out or in when I opened the screw. Hope this is more clear. 

Thanks... very clear explanation!

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