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Both will work either way, there will be a difference in lever travel and reqired pressure at the lever (how hard you have to squeeze it) depending on combination used.
78-79 KZ750 twin used a 5/8" master cylinder on single disk stock.

Post edited by: steell, at: 2006/09/17 14:59

Replacing a 1/2" MC with a 5/8" unit will significantly increase the force needed on the lever. If your front brakes are currently overpowered or close to being overpowered, you may be able to get away with it. If they're anywhere close to underpowered with the 1/2" MC, don't even think about it.

Also, don't forget about the mirror mount. Does the new MC have the same mount for the mirror?

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JMKZHI wrote:

Could you explain why it would be harder to pull the lever if a dual disk mc (5/8) is used in place of the single disk mc (1/2), while keeping the single disk setup? That seems kinda backwards to me. JMK.

Don't worry. You're not alone in this perception. In fact, there are shops taking advantage of this misperception by selling "Big Bore Master Cylinders for improved stopping power". The line pressure (P) is determined by the force on the piston (F) divided by the area of the piston (A). So, P = F/A. So, if the area of the piston increases, pressure decreases. So the 1/2" MC will generate more pressure than the 5/8" unit given the same force on the brake lever. Keep in mind that the 5/8" MC is usually applying pressure to two pistons, while the 1/2" MC applies pressure to only one. Since the 5/8" MC is driving two pistons, it doesn't need to generate as much pressure. However, it does need to move more fluid which, fortunately, it does.

That is an accurate description, but I disagree with there being a significant difference in feel between the 1/2" and 5/8" master cylinders on single disk brakes.
The 79 750 twin has a 5/8" master cylinder and single disk brake.
The 80 750 twin has a 1/2" master cylinder on a single disk brake, and has the same caliper as the 79.

I have both models, and while there is a difference in feel, it's not very much.

I will defer to steell on this one. I don't have first hand experience with this change, but I wanted to caution the OP that increasing MC bore size will increase the braking effort required. My 77 650C has dual front disks and a 5/8" MC, and it is almost overpowered, so I definitely believe that replacing a 1/2" with a 5/8" COULD work. Note I said "could", not "would". Although steell's experience indicates that it would.

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