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Help With Master Cylinder Rebuild Attempt!!

Discussion in 'XJ Technical Chat' started by day7a1, Apr 10, 2010.

  1. day7a1

    day7a1 Member

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    Background: I thought I had brake problems. I had heard that a "fountain" when you squeeze your brake is bad, and I had one. I got that idea HERE, from Chacal!. So since it is a safety issue, and my brakes seemed like they could be better, I got a rebuild kit. Didn't seem too hard, and it really wasn't.

    However, when I rebuilt the MC, I still had the fountain. I was not happy. Before writing for help, I found THIS.

    So, IS THE FOUNTAIN FROM THE RELIEF HOLE A SYMPTOM OF A PROBLEM OR AN INDICATION OF PROPER PERFORMANCE? I have to figure that out first.

    If someone could elaborate about how the system works, I can't quite figure it out. It seems so simple, that the piston pushes the fluid, but I don't see how the piston doesn't just push the fluid out the ports. The tolerances between the bore and the piston didn't seem that tight. If there weren't ports, or the ports were somewhere else, I would understand, but it seems like the pressure produced by the piston would be significantly reduced by the leak by the piston face and through the relief port and the other port....what's its name?

    Any why does the piston look like a screw?
     
  2. stereomind

    stereomind Active Member

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    It's perfectly normal to see a small fountain of fluid shoot up from the relief port when you release the brake lever. It's how the master cylinder relieves excess pressure from the calipers so the pads back off the rotors. If the relief passage is plugged, your brakes will drag.

    If you see the fountain when you squeeze the lever, you've got issues with the master cylinder, and it probably needs to be rebuilt.
     
  3. day7a1

    day7a1 Member

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    Ok, the fountain happens when I squeeze the lever.

    I did change "rebuild" the MC, I put the seal closest to the handle the way it was on the one I pulled out, that looked like it never had been rebuilt. However it looked like it was on backwards.

    Ok....so I just looked at Gamuru's Post.

    I guess I shouldn't have put the thing back together the way I found it. The rubber seal was on the wrong side of the cylinder, away from the piston. No wonder I couldn't figure out how it worked!!

    I bet I can get it to work right tomorrow.

    That makes a lot more sense.

    We all have our days, don't we!

    However, I did get the snap ring out easy by pressing down on the piston with a flat blade screwdriver (flat side down!) and then I had plenty of room to grab the snap ring. No special snap ring pliers needed!! It'll be even easier tomorrow.
     
  4. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    It doesn't push the fluid right back out the ports because as soon as the piston moves it goes past the port which is then effectively closed off to the pressure side by virtue of being on the other side of the seal.

    The tolerances between the piston and bore aren't that tight, because the design of the seals causes their "lips" to expand under pressure and they do all the work. The piston itself is more of a "guidance device" in this application.

    Not sure why the piston looks like it does.
     
  5. day7a1

    day7a1 Member

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    Thanks fitz! I'm actually not sure if it was in backwards originally or not. I took the circlip out, gave the piece to my wife to carry while I gave the tools back to the hobby shop (the Navy MWR has self service garages, it's awesome!), and she didn't keep it in the same position I gave it to her in. My fault, but it all fell out and I guess I thought it went in the other way, as I never saw the piston seal before. It was acting the EXACT same way, though. I hope didn't ruin it, and I can't wait to get back to it as I now know exactly what to do. (Trade off for all the tools you will ever need....working hours!!! Dammit!)

    Maybe Rick or Chacal knows why the piston is that way.....Hello!!....you guys out there!!!!!

    I don't know about you, but I'm quite curious.
     
  6. PainterD

    PainterD Active Member

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    I've rebuilt master cylinders in the past with no problem. Just keep track of how things are taken apart and do the rebuild in reverse. Keep things clean while working on them and you shouldn't have a problem.
     

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