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Geoff, my Sprint is a Feb '75 production with single line brakes. A few years back I replaced a leaking left rear cylinder which has let go again. On removal I see it is a 3/4" bore (it was not a genuine Girling/Lockheed part and the manufacturer is unclear). I am about to purchase 2x NOS Girling 5/8" bore cylinders - reportedly with newer seals fitted - and plan to replace the opposite side as well - which is not currently leaking. Question is: Is the 5/8" bore
going to work OK?
The problem is, they all LOOK the same! The only way you can tell the difference is by measuring the bore. The reason for the change is that a dual circuit master has 2 travel gaps on the 2 concentric pistons, making for a longer pedal. Since it would be folly beyond even Triumph's capacity to reduce the front caliper pistons to compensate, the only leeway is in the rear cylinders.
The same thing applies if you have the "Trackerjack" vented disc conversion, the bigger 54mm pistons of the Ford calipers (stock is 48mm) make for a longer pedal, fitting the smaller dual circuit rear cylinders helps to compensate for this. Of course, if you have dual circuit AND Trackerjacks like I do, you just have to live with a slightly longer pedal.
In terms of brake efficiency, you are unlikely to notice any difference using the 5/8" cylinders. The pedal bite point may be a touch higher and that's IT!
Steve