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Unable to successfully get brake lever pressure

Discussion in 'Norton Commando Motorcycles (Classic)' started by Drummer99, Jun 13, 2018.

  1. Drummer99


    Oct 18, 2013
    Hi I have a master cylinder conversion RGM I am having difficulty getting brake pressure I have bled it up with a hypodermic needle. There does not appear to be any more air coming up thru the middle hole. There are 2 holes in the ,master cylinder when I squeeze the lever I have a small amount of pressure and air bubbles come up thru the first hole. I have adjusted the plunger longer and shorter but same result. It seems like I am bringing air into the system everytime I squeeze the lever Any help here would be appreciated
  2. Tornado

    Tornado VIP MEMBER

    Dec 5, 2017
    I went through a hard time after re-sleeving my OEM master cylinder on my 850. After many days of fiddling/re-bleeding etc., turned out to mainly be due to the master cylinder primary seal already being past the smaller/towards bike centerline hole when the brake lever was at rest. Had to file some material off the lever where it made contact with the piston. Ideally, you want the primary seal sitting just a mm or two away from the smaller/towards centerline hole so that as soon as you begin applying lever movement, the seal passes that hole and therefore seals the oil in the brake line to the caliper.
    Presumably, if you bought a professionally modified cylinder, this should have already me setup right...however, if you are using a lever with a different dimension in the critical areas, then all bets are off. You need to take some careful measurements with a caliper to assess where the seals are with respect to the two holes with lever at rest.

    If you find everything looks correct, then the most likely situation is you still have trapped air....took my several more days to finally get it all out....try flicking the lever rapidly for 5-10 minutes with left, then right full steering lock (to change the caliper angle to help bubbles find there way out). When I did this, watching the two ports in the reservoir, I saw a few tiny bubbles escape...even after bleeding multiple limes bottom to top using large syringe. Another tip is to keep brake lever pulled in using elastic band with steering held at full left lock overnight...again helps trapped micro bubbles find their way up and out.
  3. dave M

    dave M

    Oct 19, 2005
    One of the issues with the RGM conversion is that the feed holes for the piston often are not at the top of the bore and you often have to remove the master cylinder from the handle bars and move it around at a different angle to purge the air from the system.
  4. Drummer99


    Oct 18, 2013
    Thanks for the replies When you are talking about the small hole to centerline do you mean the hole in the middle of the cylinder or the one closest to the wall of the M C I have the complete set up on the bench with the MC in the vice and I have tried putting the caliper flat up and now down below. I have air bubbles coming from the hole closest to the MC wall I have adjusted the piston driver in multiple positions. I have ensured that it is clear of the middle hole and when I syringe the fluid and some air comes up thru the middle hole. However I cant seem to eliminate the air bubbles coming from the other hole ? Can you explain how the 2 work together Thx Drummer99
  5. JimNH


    Jan 4, 2014
    It's air in the MC. Someone, old Brits perhaps, shows a picture of the MC removed from the bars and held vertically whilst pushing fluid up from the bottom. It's the only thing that has worked for me.

    Get a 20 or 40cc syringe and an appropriately sized tube. Home depot has the tubing for replacement of small engine fuel lines in various sizes.

    Treat the brake fluid like it was nitro, don't shake it, slowly draw it in with the syringe and then check for any bubbles. Squirt some out so the column is clear.

    Remove the MC from the bar leaving the hose connected. Dump it out and replace the cap. Hold vertically and move as allowed by the hose whilst filling through the bleeder from the bottom. Don't let any new air get in. Repeat as necessary.

    I've been surprised to find bubbles inside the syringe even when carefully drawing in fluid. Tap the syringe and get every last bubble out or you'll be frustrated.

    Reflecting on the behavior of air bubbles in the syringe I think the above procedure could be further enhanced by enlisting a helper to hold the master cylinder vertical moving it in an orbit AND tapping it with a screwdriver handle simultaneously. I've always been alone during the process but next time will have help.
  6. CanukNortonNut


    Aug 8, 2005
    You may need to tap into your M/C a little deeper. I had a similar problem with the RGM Re-sleeve Kit where it never got pressure at the lever. The sleeve needed to be seated deeper for the rubber seal to contact the bottom of the bore which stopped the fluid from bypassing around it. If all else fails give that a go. I also mark the brass sleeve with a dot so I get the bleed holes to face the top.
  7. Stillreel


    Sep 30, 2011
    I had the same problem; the re-sleeving didn't work or worked intermittently. I went with the new master cylinder instead (essentially a Honda part) and works well. As mentioned above, it would appear that the geometry of the sleeve doesn't line up consistently.
  8. Tornado

    Tornado VIP MEMBER

    Dec 5, 2017

    This diagram is useful to understand how the MC works. This is oriented as if looking from front of bike towards rear. Left side is where the brake lever pushes into the MC, right side is brake line.
    The depicted holes are closer than on the Norton MC, but same principle.
    Note that pressure can only build in the brake line when the primary cup passes to the right of the right hole. If your piston assembly is too far to the left at rest, you will not get sufficient rightward movement of primary cup past that hole. Thus minimal pressure build up.
  9. Edgarr


    Aug 16, 2016
    I too had the same issue with air in the line. I used a1/2 pint of brake fluid pumped in from the bleeder via a syringe. After that l strapped the lever to the handlebar over night. That did the trick.
  10. JimC


    Oct 12, 2007
    The way to successfully bleed brakes, any brakes, is with a pressure bleeder. Small pump up weed sprayer and a master cylinder cap with a hose nipple will do the trick, every time. Works on ABS brakes, too.

    The master cylinder resleeve sounds like a hit or miss deal. Not what I want with a brake. I went with a 11 mm Grimeca master and a 41 mm, 2 piston Grimeca caliper. Gives me a 27.79:1 ratio. Michael Morris, of Vintage Brake, says this is the ratio you want. He definitely knows what he’s talking about.
    Last edited: Jun 14, 2018
  11. pommie john

    pommie john

    Nov 18, 2005
    The comments about removing the MC are one way of doing it in my experience, but I normally choose a slightly simpler variant.

    The important thing to consider is that the air always travels to the highest point in the system. Often, when the bike is on the centre stand , the MC is tilted down a little on the right ( as you sit on the bike). This means that when you pump the lever to bleed the system, air bubbles that are sitting in the banjo on the end of the MC, get pumped a little way down the line, then come back to the banjo and never get bled out.

    If you lean the bike over onto the side stand and turn the bars all the way to the left, you can make it uphill all the way to the master cylinder so the air doesn't sit in the banjo.

    With the bike in this position you can "tickle" the brake lever and watch the air bubbles exiting into the reservoir. You can also finish the job by putting a cable tie around the lever overnight and when you release it in the morning the last bit of air will come out into the reservoir.

    One word of warning: make sure the reservoir is not filled to much before you remove the cap when th ebike is leant over.
  12. seattle##gs


    Oct 28, 2014
    What does the filler or intake port in the diagram do?
  13. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004


    "On releasing the brake(s) the return spring moves the piston back faster than the fluid can return and this causes the lip of the main rubber cup to relax and fluid passes over the cup from behind, through the holes drilled in the piston head for this purpose."

    As it's name suggests, each time the brake is released, fluid from the reservoir refills the 'main' cylinder behind the piston through the intake/filler port, replacing the quantity of fluid that passed around the piston cup.

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