Size Matters? (Rear master cylinder) (2024)

I'm fitting a custom brake set up to the MKII and have a question I'm hoping someone can help with.

I bought a Brembo rear caliper (pictured below) and was told I needed a smaller diameter master cylinder for it to work. I bought a newer sportbike mc but it will take a lot of fabbing to make it all work. Has anyone tried a caliper swap with the stock rear mc? The stock mc is 5/8" diameter. Any opinions?
Thanks

Post edited by: 79MKII, at: 2008/02/02 15:41

Think of it as a lever and a fulcrum, take a small bite with the end of the lever and you can move the weight easily, but a short distance (small master cylinder, large caliper piston), take a big bite with the lever and it's hard to move, but you move the weight a longer distance (bigger master cylinder). It's a lot easier to understand if you have moved a 3000 lb machine with a pinch bar

Another way to see it.

Assume the area of the master cylinder is 1 square inch, apply 10 pounds of force to the piston and you have 10 pounds per square inch (10 psi), now assume the caliper piston has an area of 10 square inches, 10 psi on 10 square inches equals 100 pounds of force.

If the master cylinder piston has an area of .5 square inches, then 10 pounds of force will equal 10 pounds on .5 square inch, or 20 pounds on one square inch.

That help?

Keep in mind that the caliper requires a certain amount of fluid to move it, so the smaller piston will have to move further than the larger piston to move the same amount of fluid.
Smaller master cylinder piston equals longer travel for the brake pedal/lever than a larger master cylinder piston to move the same amount of fluid.

Steel - your post was realling helping until the very end and that's where I got lost again:S

I understand the PSI part since you explained it so well, but to me that would also mean that the same lever travel with a smaller piston would result in higher psi which would result in more braking power at the caliper. The way it sounds at the end of your post is what I thought before: A smaller mc piston would require more travel to apply the brakes the same amount as a larger mc piston. If that's the case, the smaller mc would feel more "spongy", right? It there's more PSI, how can it require more lever travel?? I guess that's where I get confused. I always looked at caliper pressure as directly related to the fluid movement, which means a bigger mc would apply the brakes harder and faster. But if the caliper pressure is related to PSI, then the opposite is true, right??

Steel - your post was realling helping until the very end and that's where I got lost again:S

I understand the PSI part since you explained it so well, but to me that would also mean that the same lever travel with a smaller piston would result in higher psi which would result in more braking power at the caliper. The way it sounds at the end of your post is what I thought before: A smaller mc piston would require more travel to apply the brakes the same amount as a larger mc piston. If that's the case, the smaller mc would feel more "spongy", right? It there's more PSI, how can it require more lever travel??
because the small master cylinder piston is applying the pressure and the BIG slave cylinder is moving.(since each mm it moves it is "taking" more brake fluid behind the bigger piston to apply the brake because the bigger surface area moves then the volume of brake fluid has to equal the volume of the brake fluid moving on the small master cylinder side so to get the same volume it needs to move more cuz it is a smaller size) hope i didn't make it worse

I guess that's where I get confused. I always looked at caliper pressure as directly related to the fluid movement, which means a bigger mc would apply the brakes harder and faster. But if the caliper pressure is related to PSI,(it is related to psi, but over the bigger area so less movement to get equal volume) then the opposite is true, right??:huh

Example: (fictitious numbers)

Caliper piston requires two ounce of fluid to move from it's rest position to solid contact with rotor.

Small piston displaces two ounces over two inches of stroke.

large piston displaces two ounces over one inch of stroke.

That's the reason the lever or pedal on the smaller piston has to travel further, it displaces less volume per equal stroke length.

After the pads are in solid contact with the rotor, then next to no additional fluid is required (some, but not enough to matter). From that point on, lever/pedal travel will be equal.

If you have a solid system (no air, flex, etc), once you squeeze the lever/step on the pedal enough to start slowing down, there will be very little travel of the lever/pedal (those brakes pads don't compress much), you control the stopping by varying the amount of pressure you apply to the lever/pedal

That's where you run into the different force you have to apply to each one for equal stopping power, remember my example above? The smaller piston has double the pressure of the piston twice it's size.

It's amazing what you can do with hydraulics. I'm sure you have seen big tracked excavators on building sites or somewhere. Those are all hydraulic, that 400-600 hp diesel motor drives nothing but a hydraulic pump that's about two feet long and maybe 18" in diameter, and the pumps are variable displacement (that gets a little complicated). The operator can select low displacement for more power but slower movement, or larger displacement for faster movement but less power.

Hydraulic brakes follow the same principles

Post edited by: steell, at: 2008/02/04 00:49

I was able to pick up another KZ master cylinder pretty cheap, so I think I'll try it. Rather than having a "5/8" on the casting, it has a "14". I'm assuming (hoping) this means 14mm, which is a bit smaller than 5/8" (15.88mm)and should help. I was told to run an 11-13mm MC with this caliper, so 14 has to be close enough, I would think.

Does anyone know if the Kaws had some 14mm MC's and some 5/8?? I'm not sure what bike the 14mm one is from but it looks identical and even bolts up exactly the same as my stocker. Anyone familiar with Kaw MC's?

Single front disk 1976 KZ900 used 14mm m/c (not pictured).

Dual front disks 1976 KZ900-B1 LTD used 5/8 m/c shown here.

Post edited by: Patton, at: 2008/02/19 23:34