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Master Cylinder Plunger Binds Up

  • Thread starter Thread starter gs1000emt
  • Start date Start date
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gs1000emt

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I rebuilt my front Master Cylinder on my 80 GS1000GL but I have had nothing but issues. After I installed the new plunger and spring etc. The plunger binds up. I cleaned the bore that it slides into very well. I even took a bronze cleaning tool and hooked it up to my drill to make sure the bore was clean. Still if I put the plunger in and press it down with my finger til it bottoms out it get stuck and I have to push it back up with a screwdriver. Any ideas? This is how I have it going in there. <a href="http://s1234.photobucket.com/user/gs1000emt/media/IMG_3597.jpg.html" target="_blank"><img src="http://i1234.photobucket.com/albums/ff401/gs1000emt/IMG_3597.jpg" border="0" alt=" photo IMG_3597.jpg"/></a>
 
lubed with clean fluid. Is there anything else I can use to lube it? Although its supposed to slide nicely with just brake fluid so something is wrong. They are all genuine parts from boulevard.
 
clean brake fluid is ALL you should use as lubrication.
you say you used a cleaning tool, what was it, a wire brush type thing?
has it left the bore with a matt finish?
try polishing the bore using a small polishing mop on a dremmel or similar tool
 
Disagree with trying to polish the bore. If there are pits in the metal, get a new master cylinder.

I'd take a close look at the metal plunger piece to see if there are any burrs that could get hung up in the bore. I had to polish the plunger one time because it was out of round and binding. That was a K&L kit though (never again).
 
Last edited:
Nessism said:
Disagree with trying to polish the bore. If there are pits in the metal, get a new master cylinder.

I'd take a close look at the metal plunger piece to see if there are any burrs that could get hung up in the bore. I had to polish the plunger one time because it was out of round and binding. That was a K&L kit though (never again).
Click to expand...

i dont mean polish it, as in mirror polish, just polish it smooth if the drill tool he has used has left a rough finish! :)
 
I used a bronze wire brush inserted into my drill. But I only did that after the initial problem with it binding up. So if the Master is bad? Where can I purchase another? Do I have to buy aftermarket? Im on a pretty tight budget.
 
I'm concerned that the bronze brush might have scratched the bore. Shine a light down in there and try to feel it with your pinky, if it doesn't look and feel perfectly smooth, it's no good.

I recall when reassembling my master cylinder with new parts that the piston was a little sticky at first, even with copious amounts of fluid coating everything. After it was installed and properly bled, however, there was never any sign of binding or stickiness.
 
This is the correct order for the parts. Left is toward the banjo bolt, right is toward the lever. Substitute the plastic washer and wire clip on the far right for the new dust boot.

20130106_135921.jpg
 
eil said:
I recall when reassembling my master cylinder with new parts that the piston was a little sticky at first, even with copious amounts of fluid coating everything. After it was installed and properly bled, however, there was never any sign of binding or stickiness.
Click to expand...

Were you able to do something to correct this "stickiness"?
 
Sorry dont know why the link wasnt working. But yes that is the order I have it. Any suggestions on replacement masters?
 
Ok I just got off the phone with a local Machining and Fabrication Company. They said they can take an initial measurement at the opening of the bore and run a boring tool in to clear out the master and make it precise. Do you guys think this is a good idea? Does the bore stay the same measurement from the opening to the bottom?
 
Not sure, but here's a thought. Maybe it's only getting stuck because you are pushing it in much farther than it was intended? Perhaps the rubber bits get jammed and they get stuck? It never bottoms in the bore in normal operation, maybe it's not intended to.
 
Well tkent i thought that too so i put it together and did not press the plunger down and put it on the bike. i pulled the brake lever to bleed and it immediately bottomed out and got stuck. it will return if i just press it down a little by hand but as soon as i press it further it gets stuck. This bike sat for about 10 years and had a lot of gunk in the mc when i cleaned it. im thinking there may just be a tiny bit of rust catching the plunger. ive tried everything to clean it up but im thinking honing may be my last resort. Id like to save this mc if possible cause only replacements i can find are chinese cheapies.
 
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