Andover Norton have the repair kits for the Lockheed front brakes.
The repair kit for the caliper is CP2696-RK. For the two ring master cylinder fitted to twin discs it is CP2410-1RK and for the single ring single disc master cylinder it is CP2215-20RK. Make sure you order the correct kit if it is for the master cylinder as they are expensive now. Andover Norton are very helpful if you call them.
Its for the twin disc master cylinder all I need is the inside piston seal.
The kits seem to contain this one wakes and the external boot and cost around £70!
If I check triumph spares on line you appear to be able to buy kits for £20 or just the seal.
I know - I would have to know which one fits!
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The seal kit is expensive and I don't know of any alternative. RGM Norton have the seal kit £42.50 + vat which is a bit better than Andover Norton. You can buy an after market complete master cylinder for less than the seal kit. Suppose it is the price we pay for having long discontinued equipment. A smaller bore master cylinder would give a brake with more feel. Quite a few Morini and Norton owners are using 14-16 mm master cylinders with similar sized calipers pistons and report a better brake. Probably require a new brake pipe though so the cost goes up!