Front Brake Master Cylinder Upgrade

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cterry66

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I decided to change the Front master on my 75 MK3 rebuild to a Triumph America / Bonneville master and switch unit. Still using the stock brake caliper that is newly rebuilt with seals and pistons and has a brand new braided SS line. The Triumph unit has an 11mm piston and after much bleeding, top down, bottom up, vacuum pump, it at best feels mushy. Tried a second identical unit with a rebuild kit in it...exactly the same results. Put a very used Magura 13mm unit on, it bled immediately and had a good solid feel.
Is the 11mm master too small to provide enough volume of fluid to extend the caliper pistons?
I have a lot of time into the Triumph unit having already modified the wiring harness to work with the stock Norton setup and that all works perfectly so dont want to give up on this project.

Charlie
 
Now it seems solid on the magura, try swapping. Back to triumph. Perhaps you finally got some air out of the Lockheed caliper. These seem to be very stubborn to clear. Took me ten days of trying different methods to firm up my mkii with a resleeved Stock MC. Ultimately in frustration I started rapidly flicking the MC lever continuously for 40 or 50 time while watching the fluid in reservoir. Suddenly saw a few tiny bubble float up, then another, then several more. System was then firm.

A vacuum bleeder may be the best method on these old calipers.
 
I left the caliper off, and rolled it around in my hand whilst pumping the lever. Eventually, all of the air rose to the surface of the reservoir; maybe fifteen minutes of lever flipping. I didn't bother with a vacuum purge (cheapskate/Scottish).
 
I removed the Magura Master and tried both of my Triumph units and neither would build pressure adequately.
I have a Mity vac and tried it many times without success. I also tried injecting fluid into the bleed screw on the caliper pumping the fluid up to the master. I primed the caliper with fluid before I connected the line up as described on OLD BRITTS website. I tried both of my calipers to see if they were the problem. All methods so far seem to purge the air bubbles and then I get clear fluid flow.
 
I just did the math and checked the "Vintage Brakes" Ratio chart. Looks like a 12mm unit would be optimum for the Norton caliper.
Hard to imagine that 1mm would make that much difference in the feel at the lever. I'm guessing now that I do have my brakes bled properly and the mushy lever feel I'm experiencing is a function of the less than optimum Master piston diameter.
 
That mushy feel has nothing to do with piston diameter. Air compresses, brake fluid does not.
 
Taking the brake pads out and pushing the pistons back with a tyre lever has served me well over the years, but a Mityvac is a worthwhile investment.
I've found that old brake hoses can hold air too, and tapping along the hose with a screwdriver helps.
Replacement is the 'correct' answer...:)
 
I have a 12mm Brembo master cylinder and it makes the stock Norton brake feel like a modern brake. It seems to be the ideal size. It is strange that they didn't come this way as it is such a huge improvement. A friend of mine has a very special bike we call the Impfield. He has dual Lockheed aluminum calipers and his own home made master cylinders. He said he was never able to bleed the brakes perfectly until one day he had a small tip over, the bike ended up upside down and once the bike was righted the front brake felt better than it ever had before!
 
I had a 11mm Yamaha XT cylinder on my Mk3 and it worked great, maybe a little too strong. This was just for a trial. I resleeved down to 1/2" diameter, made my own piston, used Harley seals, and kept the stock appearance. I believe you still have some air in your system.
Jaydee
 
JimC wrote: "That mushy feel has nothing to do with piston diameter. Air compresses, brake fluid does not."
Well, not exactly true. The original master cylinder was too big, which is why everybody refers to it as "wooden". When you squeezed the brake lever, it didn't move. As you reduce the size of the master cylinder, even with all the air out of the system, you will end up with some "squish". If you go too small in piston diameter, you will have too much squish and the lever will run out of travel (as in hit the bar). Some squish is okay. Where does it come from? Usually from the hose expanding, would be my guess.

Stephen Hill
 
peter12 said:
I believe an 11mm piston might give too much lever movements.
Click to expand...

I use a racing Lockheed master cylinder which is intended to be used with one Lockheed calliper to operate two similar callipers. There is a bit more travel but it is not excessive. However when I am racing, I hold the twist grip with three fingers and a thumb and operate the front brake with one finger. I can stop the front wheel with that one finger and for racing, it has to be that way. You can only go as fast as you can stop.
 
Stephen Hill said:
JimC wrote: "That mushy feel has nothing to do with piston diameter. Air compresses, brake fluid does not."
Well, not exactly true. The original master cylinder was too big, which is why everybody refers to it as "wooden". When you squeezed the brake lever, it didn't move. As you reduce the size of the master cylinder, even with all the air out of the system, you will end up with some "squish". If you go too small in piston diameter, you will have too much squish and the lever will run out of travel (as in hit the bar). Some squish is okay. Where does it come from? Usually from the hose expanding, would be my guess.

Stephen Hill
Click to expand...


That ain’t what Pascal said. Granted, there are other contributors to squish in a brake system, but my contention was there was still air in the system Cherry spoke of. Going to a smaller diameter m/c piston will increase the squish feeling, but the force on the caliper piston will be greater.
 
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When my Norton tired to kill me and threw me over the handle bars from stock brakes failure and while recovering from my first ever broken bone, in my arm I replaced all my front brakes with a full Grimca set up, 12" floating disc, Grimca race calipa and Master cylinder and SS brake line, that was over 7 years ago now one finger use if I want to and my brakes work great, at the time cost me just on $500 from RGM and one of the best improvements ever and it looks great on the Norton, my life is more important that keeping the stock brake set up, but them my bike is no longer stock.

Ashley
 
ashman said:
When my Norton tired to kill me and threw me over the handle bars from stock brakes failure and while recovering from my first ever broken bone, in my arm I replaced all my front brakes with a full Grimca set up, 12" floating disc, Grimca race calipa and Master cylinder and SS brake line, that was over 7 years ago now one finger use if I want to and my brakes work great, at the time cost me just on $500 from RGM and one of the best improvements ever and it looks great on the Norton, my life is more important that keeping the stock brake set up, but them my bike is no longer stock.

Ashley
Click to expand...

I, too, had a Grimeca master cylinder and Grimeca 2 piston caliper with a 12” floating disc that was a 1 finger stopper. The caliper piston diameter was 41mm, the m/c was 11mm. This gave a piston area ratio of 27.79:1. The sweet spot Vintage Brake spoke of. This was no less of a brake than the dual disc Brembo on my BMW.
 
JimC said:
I, too, had a Grimeca master cylinder and Grimeca 2 piston caliper with a 12” floating disc that was a 1 finger stopper. The caliper piston diameter was 41mm, the m/c was 11mm. This gave a piston area ratio of 27.79:1. The sweet spot Vintage Brake spoke of. This was no less of a brake than the dual disc Brembo on my BMW.
Click to expand...

I fully agree with you Jim, I had twin Brembo's on my BMW and my Norton with the Grimeca set up work as good as the Brembo's and after 7 years or more now have had no problems at all with this set up, but then I been riding long enough and not a heavy user of my brakes although I ride my Norton hard and always pushing it to its limits.
The best thing is the Gremica set up is a way lot cheaper than the Brembo set up and works as good and it looks good as well.

Ashley
 
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