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no master cylinder pressure after rebuild

  • Thread starter Thread starter scottsgs
  • Start date Start date
S

scottsgs

Guest
Hi all,

I really need some help here. I recently rebuilt my rear master cylinder because my rear brakes started to drag severely. I went to Alpha Sports and bought a rebuild kit. I followed the directions in my Clymer manual to the letter, but I cannot get any pressure to build up in the M/C. I also recently bought new pads as they were in bad shape. I looked up the exploded view on Alpha Sports web site, but it is hard to tell what goes where. The guy at Alpha Sports said they don't rebuild M/Cs they just replace them.

I am going on a month without being able to ride. It seems like no one has any info that will help.

thanks,

Scott
 
Assuming you have assembled the master properly and there is nothing wrong with the bore causing a leak the most likely issue is you need to prime the master. Crack the banjo and try to get some fluid in that way. Some people fill up the master off the bike while holding the outlet port straight up. Not sure what to tell you about the brake pad assembly, I don't remember it being all that hard. If you post a photo of the parts you have someone will be able to help.
 
The easiest way to prime the master cylinder is to open the bleed nipple, and use a syringe with a piece of tubing to suck brake fluid through the system.

Once you've got brake fluide coming through into the syringe, you can then bleed the system of remaining traces of air as per the usual method.
 
I had a similar problem after re-building a front master cylinder. I was eventually able to get the system primed by filling up the master cylinder off the bike and forcing fluid through the lines with a syringe, but the real problem was that the bleeder port in the M/C that releases the pressure when the brakes are off was plugged.
Once I fixed that, I still had to prime it manually, but it primed much more easily. Been working fine ever since.
 
Forgot to mention in the last post . . . reason I'm suggesting a blocked bleeder port in the M/C (if priming alone doesn't fix it) is because you mentioned that your rear brakes were dragging and you rebuilt the master cylinder, not the wheel cylinder. So I assume that the piston(s) in the wheel cylinder are sliding easily and you decided that the M/C was the problem. Before you rebuilt it, was the brake pedal also 'hard', as if the pressure in the system wasn't being relieved? Did you check that relief port to see if it was clogged? It is very, very small.
 
Another common reason for why brakes drag, is that the piston seals in the caliper are old and have lost their zing (highly technical term, that), and so the caliper pistons do not retract that extra poofteenth (another technical term) to allow the pads to back off from the disc.

Replacing the piston seals is easy to do, and it certainly solved the same prob. I had with my GS1000S.
 
Return hole

Return hole

I rebuilt a front M/C once, and did not see the return hole for the crud that completely filled it, and also couldn't get it to flow. Once my error was pointed out, and remedied, everything worked fine. Might want to look for a similar situation.
 
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