Commando master cylinder re-sleeve

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Jul 19, 2003
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I have a 73 Commando with a Hyde racing front caliper, a 13" Hyde disk and a stock master cylinder that was recently resleeved with a stock 15mm piston. The braking performance of this setup is pretty bad and requires a huge amount of effort on the brake lever to get adequate stopping. Additionally, the lever has a rock hard feel with very little sensitivity.

I have heard that resleeving to a 12 or 13mm piston in the master cylinder will provide more mechanical advantage, which will require less effort to stop and also give better feel. Does anyone out there have experience with this type of a resleeve? If so, please let me know how it works and where you got it done. I do want to retain the stock master cylinder for looks.

I also just read that some racing calipers actually use a smaller diameter piston than the stock unit. In this case, more lever pressure will be required and by using the Hyde caliper with the 15mm piston, I may have a set up that actually requires more force to stop than the stock set up. Any thoughts on this?

Thanks for any help you can provide.

Mike Grant
Front Brake - Master cylinder

What are the symptoms of a worm front brake master cylinder?
I bought my 73 Command with standard front brake master cylinder and a 4 piston front brake caliper. The braking performance is bad. The lever can come back to the bars.. pumping is required.

I bled the brakes but the problem remains. I was bleeding the brakes according to the manual but squeezing the lever without the top on the reservoir (and the bellows) sends fluid squirting out.

I suspect the Master Cylinder is worm or the seals shot.

Any comments?

Thanks Dave. I'm off the road until this is sorted.

The bike came with the standard caliper in a box... Either I return the brakes to original or source another master cylinder. I wil probably do both... As I don't have any specs on the new caliper I need to remove it & measure anyway. Thanks for the links.
Std caliper and master cyl

A rebuild kit is available for the standard caliper for about $25. It has new seals, and is easy to do. Use compressed air into the brake fluid hole to blow out the pistons if you cannot pull them out. Rebuilding makes a *big* difference in stopping with the stock caliper. This is the most common source of poor front brake performance.

You can also rebuild the master cylinder with new seals and a plunger, which is good because the plugers get pitted after 30 years....

That said, stock caliper and master cyl are pretty weak, even after a rebuild. It will go from "dragging feet on the ground to stop" to "sort of stops, with some effort".

I put a new dual-brake AP Lockheed master cyl onto my stock caliper, and I found that feel was FIRM and much improved from the stock master cylinder, however actual braking performance wasn't hugely improved. Note: I also ground and drilled the stock single front disk... looks cool and stops marginally better.

Then I upgraded to dual Norvil 12" disks and used the dual-braked AP master cylinder.... Woah Nellie... I got too much brake. I was able to lock the front brakes using 2 fingers at 80mph....

What I was recommended by Les Emery at Norvil, and what turned out to be the hot ticket, was the stock master cylinder with the dual disk brake calipers. Beautiful feel and tons of brakes....

I now have the stock caliper and the new dual-disk master cylinder for sale... :)

After removing my after-market front brake caliper and measuring the pistons I appear to have 4 x 30mm pistons with an area of 2827 sq mm. By looking at the 'Front Master Cylinder to Wheel Cylinder Ratio Chart' the recommendation is to use around a 12mm master cylinder (25:1).

My Norton Commando Workshop does not say what the size of the standard master cylinder or brake piston(s) are. I assume that the standard master cylinder is 5/8 inch while the brake pistons are 2 x 41mm or 44mm. I have't disassembled the std caliper or master cylinder yet. Either way the standard configuration and my current setup have a ratio of 15.46 and 14.28 respectively - neither look right.

Any comments before I spend money on a new or re-sleeved 12mm master cylinder?
master cylinder

The stock master cylinder is 5/8 bore and the stock caliper pistons are 44mm. the ratio is appx. 15-1 and this is why the stock brake SUCKS.I have resleeved my master to 13mm and with a stock caliper you get a ratio close to 24-1 for a major improvment.
Another bike that I am working on I orderd the 13" lightweight single rotor kit from NORVIL. When I orderd it I asked them what mc. bore they reccomeded to use and was told the stock one. WRONG!!! the AP caliper that it came with has 41mm pistons and with the stock mc. you will endup with a ratio close to 12-1. looks like there combo would be a step backwards. I used a 13mm mc because I could not find and was told nobody does a 12mm one.

master cylinder update

I just got back from vacation and wanted to thank everyone for their posts and also give an update on what I plan to do.

I found the Vintage Brake table to be very useful (thanks, Dave) and it helped explain why I have such lousy brake performance. As other posts indicate, it all seems to come down to the ratio of the master cylinder piston area vs. the combined area of the caliper pistons. With the current set up, my ratio is less than 14:1. The Vintage Brake table suggests that 23 - 27:1 is probably best. So, clearly, my setup is suboptimal.

With the racing caliper and 41mm pistons and a 13mm master cylinder (vs. the 5/8" stock), the ratio is close to 20:1, which is getting to where I want to be. Then, with the 13mm m/c and the stock caliper and the 44mm pistons, the ratio will be ~23:1, which would be better. I do have the 12" Hyde disc, which will provide some more mechanical advantage compared to the stock disc, so maybe the racing caliper will be okay.

First, I plan to get the m/c modified. Both Vintage Brake and Fair Spares will do this to the stock unit for around $125. I will try this with the racing caliper and see how it works. Then, if that is not enough of an improvement, I will go back to the stock caliper and try it.

Anyway, it will take me a few weeks to get this completed, and I will post the results as I go along.

Thanks again for all your information.

Mike Grant
Braking News

Rather than re-sleeve the orginial master cylinder I am getting a Grimeca 13mm M/C with adjustable lever. Britishspares are asking $NZ148. This sounds better value than re-sleeving. I was after a 12mm M/C but I can't find one.

The 13mm M/C with the standard caliper gives me a braking ratio of 23:1. Not perfect but a lot better than the 14:1 I started with using the non-standard 4-piston caliper.

I will keep the standard M/C in case I go to a dual disk setup.

When it comes together I will update the forum.

master cylinder

From a functional and price standpoint, the new Grimeca m/c is the way to go. You get a purpose built-new component that will work very well at a price that is similar to a reworked original component.

I wanted to retain the stock mirror, switch gear and brake switch, so I will try the re-sleeve.

Looks like some of us will have better stopping Nortons in a few weeks.
Things are not working to plan.

The Grimeca 13mm Master Cylinder requires adapter plates to fit the switchgear. Britishspares provided them - thank you. It doesn't look pretty - you might call it functional.

The master cylinder does not have a fitting for the brake light. I thought it would be easy to get a banjo bolt with a built-in pressure switch that fits but that is proving difficult. The only ones I found have a course metric thread.. where the Grimeca is definitely a finer metric thread.

Rather than have the bike off the road I rebuilt the original M/C but either the bore or plunger is worn.

I'm off to the local Ducati restoration shop for guidance. The early Ducatis had pressure switches on their brakes so I'm hopeful to find a part that fits.

I'm keen to hear the results of the re-sleeve option as I may yet get it done. Do they provide a new plunger and seals as part of the re-sleeve service?

I have not gotten my M/C back yet from Vintage Brake with the 13mm resleeve. I am expecting it next week. Since the piston size is changing, all of the seals and internal components will be new.

I considered the Grimeca when I restored the bike but rejected it for the reasons that you mention. You lose the stock mirror mounts, the switch gear and have to cobble up some brake light switch. I saw one on a bike and didn't like the looks at all. However, I am sure that the performance is fine.

I asked Vintage Brake about the stock caliper vs. the Lockheed unit. The stock caliper uses 44mm pistons and the Lockheed uses 41mm, so the stock caliper would provide a better ratio. This is true, but according to Mike Morse of Vintage Brake, this benefit is probably overshadowed by the fact that there is a much greater selection of pads for the Lockheed. The standard caliper has only a few compounds available, all of which are not good. I am going with a Platinum Ferodo pad, which is supposed to be a good compound for the street and will provide the initial bite that I am looking for.

I will also change the rear shoes to a Ferodo lining and will post on the results. The current performance of the original shoes is horrible, almost non existent, but I suppose that is to be expected with friction components that are 30 years old.

I'll let you know the results as soon as I have something.

More Brakes

I now have a Grimeca 13mm Master Cylinder & lever with a Magura mechanical brake light switch. Very neat. This is still using the standard switch block with some adapter plates. Next component is a mirror.

I tried a banjo bolt brake light pressure switch but it hung down too far and potentially fouled the tank - or at least very close.

Although the bike came with a 4-piston caliper I was not happy with the braking performance. It was particularly ugly .. made for a Harley I think. The mis-match between M/C & brake piston ratios has been well covered.

When I re-installed the standard caliper I could not get a firm pressure on the lever. It looks like the std caliper has a frozen piston. I probably should have overhauled it anyway.

Any hints or tips on a front caliper re-build? I see I need a 'peg spanner'. Is this a commonly available tool or am I going to pay Norton prices for it?

All my previous bikes were Japanese ... I'm on a steep maintenance learning curve.
More Brakes


There are a lot of stock caliper rebuild kits on the market - aftermarket stainless steel pistons are probably better than the stock units and seem to cost about the same.

It is easy to build a peg spanner; any fairly heavy piece of bar stock will do - drill two holes at the end at the same spacing as the holes in the caliper plug and thread a short bolt and nut through each hole (I made the holes in the bar stock slightly oversize to provide some play, centered the bolts in the holes in the caliper plug, then tightened down the nuts). The adjusting spanner provided with my Hagon shocks worked well for the bar stock.

Caliper Rebuild

Daniel - Thanks for the advice.

I built a peg spanner as you recommended and partially disassembled the caliper. I say partially because the inner piston is recessed and stuck fast.

Any advice on how to dislodge it? It wouldn't pump out using the master cylinder (with brake fluid) and air pressure didn't work.

Other than drilling a hole in the inner piston & dragging it out I don't know what to do next.

I finally got all the components working & assembled.

The combination is:

. European Handlebars (narrow)
. Grimeca Adjustable 13mm M/C (with adapters for the standard switch block)
. Magura mechanical brake switch
. Standard brake hose
. Standard from caliper (reconditioned replacement)

The only part of the front braking system left from the bike when I bought it is the disk roter.

With the narrower handlebars the brake hose does not foul the tank. The Grimeca Master Cylinder is adjustable - which is excellent for removing 'slack' from the lever. The mechanical brake switch screws in easily and I have run new brake wires into the headlight where they plug into the original wiring.

I bought an off the shelf mirror for $30 with a metric thread and used only the stem. The Norton mirror (reflector) fits straight on. In fact the stem is higher and a big improvement on the orginial.

So the braking works and it has some feel to it. I will ride it for the next couple of months to see if it is enough.

At this stage I think I will be looking for more.

The AP Racing Caliper (44mm) looks expensive but could be the next step.

When you don't know what you are doing it gets expensive.
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