Topic: Slingshot Master Cylinder not pushing fluid through the line..

[CapeCafe]

Hi - I have a 76 CB550 with a Slingshot master cylinder (MC) and stainless lines.  I changed the front brake pads yesterday and when I went to bleed the brake after reinstalling the brake line, I'm not getting any fluid pushed through the line to the caliper.  The master cylinder is just three years old and has always worked perfectly as designed.  With the MC full of fluid I am only pushing air out of the end of the brake line with a bit of fluid sputtering through when I work the brake lever.  I have attempted to crack the banjo bolt at the MC connection to see if there may be a bubble there stopping the MC from priming.  Didn't have any effect on the problem.  Any thoughts on what I'm not doing correctly or what I should be doing to get the MC primed and pushing fluid to the caliper?  All help is appreciated.

[CapeCafe]

I can use my vacuum bleeder and pull fluid from the bleeder valve on the caliper to the master cylinder (MC) and observe the fluid level drop in the MC.  I refill before it gets too low, cap it, pump the lever in short bursts, hold the lever to the throttle grip and crack the bleeder valve until no air exits and a steady stream of fluid flows.  Close off the bleeder valve and still no pressure on the lever or movement of the caliper piston after multiple tries; same result.  Can I be loosing pressure at the caliper piston seal?  there is no fluid leaking out around the piston. It is completely dry around the bottom of the pad.  I did pull the piston, clean and re-grease the caliper bore and piston.  I also attempted to bleed any potential air locks at the banjo bolt connections by cracking them until fluid shows then locking off quickly.  I didn't have this issue during the original install three years ago

[scottly]

Try this: remove the caliper adjuster screw and spring, and then griping the rotor with your fingers, apply pressure with your thumbs on the caliper body. This should slowly force the piston back into the caliper, and force any trapped air upwards instead of downwards, bleeding out through the tiny hole in the MC.

[CapeCafe]

Thanks Scottly.  That helped to firm up the brake lever.  I appreciate the tip.  While the lever is at full stop now about 1.25" from the throttle grip, It still seems it could use some more bleeding.  I don't remember how much play I had on the front brake lever, but I do remember it being firm.  It may be I was not bleeding in the correct sequence?  It's been a couple years since I've even ridden the bike and I could be hallucinating that a 43 yo single side and single piston front brake is as firm as the new Brembo pinchers I ride on regularly   After pushing the caliper piston all the way in, I filled the MC reservoir the short pumps of the lever and a crack of the caliper bleeder valve until fluid appears (right away) hold to grip and lock off.  I then repeated the same and cracked the banjo bolt at the MC until fluid appears (right away).  One last repeat back at the caliper bleeder valve and it seems to be working well.  Thanks again Scottly.