Rebuild a Norton Commando Disc Brake Caliper

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Old versus new: A 1974 Norton Commando 850 disc brake caliper stripped bare with old parts on left, new on right. 

Although some people today criticize the front disc brake fitted to the Norton Commando starting in 1972 (earlier models used a twin-leading-shoe drum) for a lack of bite, they were highly regarded when new and considered superior to the drum brake setup.

It’s important to remember that 40-plus years of development has resulted in performance levels in 2014 undreamed of in 1972. Former Norton employee and now Barber Museum restoration expert Brian Slark notes that when Norton introduced its disc brake, “Britain still had streets paved with cobblestones and wood blocks sprinkled with tramcar lines everywhere, and the last thing you wanted was to send a rider skidding down the road when he squeezed the front brake.”

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That doesn’t mean the Norton disc brake was without issues, however. Similar to many bikes, disc brake model Commandos were originally equipped with chrome-plated iron pistons. The problem is that over time — and especially if moisture gets into the system or the brake fluid gets contaminated — the chrome on the iron pits and flakes, wrecking the caliper seal. Further, the steel caliper piston plug fitted to the Norton caliper tends to seize to the caliper body. The cover is screwed in tight using a “peg spanner” and the pegs — or holes — in the cover are often damaged during removal.

Stainless steel, however, won’t flake, and it’s extremely resistant to corrosion and pitting. That makes it a perfect material for replacement caliper pistons, something the aftermarket has long recognized. Norton didn’t specify stainless steel pistons, most likely because they would have been more expensive.

For this How-To, we rebuilt the front brake caliper on a 1974 Norton Commando 850. Our caliper pistons were corroded, with the chrome surface scuffed and pitted, so along with new caliper piston seals we installed new stainless steel caliper pistons and a new stainless steel caliper piston plug. We sourced our stainless steel pistons and plug from Job Cycle, along with a rebuild kit containing new caliper seals and plug seal, and a new stainless steel bleeder screw. Total cost was $146, not including brake fluid. The brake pads had been recently replaced, so we reused ours. Budget another $30 or so if you need new pads.

You’ll need a “peg spanner” to remove the outer caliper piston plug. To learn how to make your own cheaply and easily, click here.

This is a fairly straight-forward job, easily within reach of the average weekend warrior. Although a seasoned wrench can complete the job in a few hours, we’d suggest budgeting a full morning to give yourself time for the invariable problems that come with working on old iron.

There’s only one special tool required, and that’s the peg spanner for removing and installing the caliper cover. The factory tool sells for around $90. As always, we suggest having a good shop manual on hand to help aid in the identification of critical parts and for proper torque specs.

To begin, remove the brake caliper. Start by unscrewing the hydraulic brake line, using a line wrench as shown to avoid rounding off the line nut. With the line free, it’s a simple matter of removing the two bolts securing the caliper to the fork leg, followed by slipping the caliper off the brake rotor.

Secure the caliper by its mounting lug in a vise. Sandwich it between thin wood shims or rags if needed to avoid scarring the caliper mount. Next, remove the caliper plug. It’s typically stuck fast and ours wouldn’t budge at first, so we used a hand torch and gently heated the caliper body.

With the caliper heated up, the plug came out easily using a peg spanner. This was our first chance to use our homebuilt peg spanner, and it worked perfectly. Under the plug is the outer piston, which we’ll remove next.

Using finger pressure or a large C-clamp or other suitable tool, push the outer piston far enough back into the caliper to expose the caliper piston seal. Using a pick, pull the seal from out of its seat, being careful not to scratch the piston bore.

At this point the piston should come out easily. The stock piston is chrome-plated iron, so we just used a magnet to “grab” it and pull it out. If the bore is really dirty, clean it with brake fluid and a green Scotch-Brite pad to get the piston to slide out easily.

The inner piston is a little harder to remove. Stuff the bore with a rag to act as a cushion. Plug the transfer port to the rear piston with your finger. Using compressed air and a blow tip, apply air pressure at the line fitting in short bursts, easing the piston out slowly so it doesn’t pop out suddenly and hit your finger.

The inner piston comes out with its open end exposed. If a magnet won’t pull it through, try using a pair of needle-nose pliers, spreading them out to grab the inside of the piston to pull it out from the caliper.

Once the inner piston is out, remove the inner seal with a pick. Next, remove the bleed nipple then thoroughly clean the caliper using brake parts cleaner and a green Scotch-Brite pad to remove all traces of contamination. Inspect the bore for damage, making sure it’s clean with no signs of dirt or residue.

Now it’s time to start putting it all back together. Before we installed the new piston seals, we soaked them in brake fluid to lubricate and condition them. After making sure the caliper is absolutely clean, install the inner seal in its seat in the caliper.

With the inner seal installed, lightly coat the inside of the bore with brake fluid, then place the new stainless steel piston into the bore, flat side first, pushing with finger pressure only to get it past the lip of the seal.

The piston will usually push home with finger pressure, but if you’re worried about it going in straight secure the caliper to your bench vise, then use a large C-clamp to gently push it home. The piston need only go far enough for the brake pad slot to be fully uncovered.

With the inner piston installed, install the outer piston seal. Here you can see the inner piston in place. Once the seal is installed, lightly coat the bore with brake fluid and place the outer piston into the bore, flat side out.

Push the outer piston in with finger pressure or using the C-clamp, again making sure the brake pad slot remains fully uncovered so that the pads, when installed, are as far back as they can go.

Next, install the new seal on the new stainless steel caliper plug. It only goes on one way, but make sure it doesn’t wad up or twist in its seat so the plug will seal properly when installed.

With the plug seal installed on the plug, screw the plug home into the caliper body, using the peg spanner to ensure it’s fully seated. It doesn’t need to be super tight; properly seated it should be flush with the caliper body.

Next, install the new stainless steel brake bleeder nipple. Here’s our caliper with all parts installed except for the brake pads. Install the pads keyed into their slots, metal side toward the pistons. They’ll slip out of the bore easily until the caliper is installed; keep them in place by wedging a short piece of rubber fuel line between them.

Install the brake caliper on the fork leg (removing the rubber fuel line used to wedge the pads) and tighten it to spec, followed by the hydraulic brake line. Flush fresh brake fluid through the system. To ensure all the air is out of the system, pull the brake lever back and strap it down overnight.

The next morning, remove the strap and check the brake fluid level, topping off as necessary. If you installed new pads, it’s a good idea to go out for a ride and bed them in, running up to 45mph or so several times, pulling hard on the front brake to heat up the pads. Follow this with several miles of cool down. That’s it, you’re ready to go!

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