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Front brakes Don't receive pressure until 3rd PUMP!

Jose

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Help me out guys. If i need a whole new master cylinder let me know they are kinda cheap anyways. Obviously the resevoir of my 1981 cb750c fluid is at an angle but when I hook on for braking power it either doesn't work it takes me 3 pulls for it to actually stop. What should I do? Look for the faulty line? Replace the whole master cylinder or maybe bad calipers?
 
Most likely but brake issues are important enough that others could do well to see it.

On site after site I am constantly amazed at the numbers of people who being wrapped up in their own issues do not consider the bigger picture when brake operation is involved. Still insisting on driving vehicle like that is literally taking your very life (or that of others) or future in your own hands.

Time to stop and think there, then, or now.
 
Looks like this thread has been sitting for a bit, but I have a similar problem. '76 CB750K. I just rebuilt the front caliper (purchased new "caliper A" with new piston, seal, pads, bleeder screw, and new brake tubing). I bled the brake, using a piece of clear plastic tubing attached to the bleeder to watch for air and fluid. Pumped the brake lever until it was hard, held it down, opened the bleeder, watched air and fluid come out, closed the bleeder, released the lever. Kept the reservoir full. Did that many times, until no more air bubbles came out. Now I pull the lever and I get about two soft squeezes in before it firms up, and even then it doesn't seem to be applying much pressure to the rotor. Is it possible I didn't finish bleeding? Should I try the reverse pump method or syringe?

Another thought, the replacement piston was hollow and I didn't put a cap on it. So it's a bit shorter than the original piston. Is it possible that the new piston is too short to grab the rotor? Seems like it would be forced against the rotor regardless. Any thoughts welcome. Thanks!
 
You're trying to bleed air downward, which is contraindicated in all of nature, air bubbles go up. To add to it, the total length of M/C piston volume movement there is WAAAAY short of purging the entire line length volume, meaning you SHOULD be encountering problems......................
 
Try pumping the lever, open the bleeder screw and then quickly close the bleeder screw right before the lever bottoms out. Do this several times. Worked on my CBR with the same problem.
 
It can work but often doesn't, every time you stop for a second the bubbles move back up higher in the line to frustrate you. The bubbles don't simply stay glued in place where they last were while you do the work. Reverse bleeding from the bottom up works well for some people. Or, vacuum bleed at caliper, that action works fast enough it can override the bubble rising tendency.
 
I had that problem when I rebuilt my caliper. Take the caliper off the fork, still connected to the master cylinder and hang the caliper from the ceiling so it hangs higher than the master cylinder. Don't forget to put a piece of wood in between the pistons and pump the brakes. Bleed it like that a couple of times and let it hang for a day.
The bubbles will make their way up to the caliper. Bleed it again and the line should be clear of air. It worked for me.


 
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