Need Help with Master Cylinder Swap

I have a 1986 VFR700 with a CBR F2 front end swapped onto it, including the F2 brakes and I believe brake/clutch master cylinders off an F2 as well.

I want to swap the master cylinders. The old-school metal units are corroded, they're a pain, don't hold a lot of fluid, don't refill easily, etc...I'd like to swap to newer plastic reservoir units.

Has anyone ever done this before? If so, what master cylinder size do I need to ensure correct braking on my F2 calipers and correct clutch actuation on my VFR700 transmission? Will cylinders from a newer VFR work, of maybe an F4?

My apologies if this has been covered before, I couldn't find mention when I searched. Thanks!

Look on the bottom of your current master cylinders to find their I.D. cast into the body.

They could be metric, so it might say 14 or 16 perhaps.

Or they could be standard and say 1/2" or 5/8"


Once you know that, find units from newer Honda sportbikes with the same internal diameter and the plastic reservoirs you want and you're all set!

Thanks man! Why is it always the simplest answers that elude us? Or maybe just me. Anyway, you rock.

If your calipers and Master cylinder are actually from an F2, then the master cylinder is 1/2", or 12.7mm, ID. Unfortunately, I know of only ONE modern Honda with a remote-reservoir 1/2" ID master cylinder. The CBR400RR NC29. Not too many NC29 parts over there, I'd imagine.

You could go up a size (the next larger would be the ultra-common 14mm), but you will change the ratio. A larger master cylinder gives better feel, at the cost of less power, so you will have to squeeze harder to stop as quickly as you would with a 1/2" master. Many people like this and do it on purpose.

Ciao,

Looks like my master cylinder isn't from an F2 then...it's 5/8. Clutch cylinder is 14. Are those stock VFR?

So I've been running a MUCH bigger nearly 16mm. I do kinda like the feel, but I had noticed it took a pretty heavy wringing of the lever to haul the bike down. Maybe I'll switch to the 14mm then...a compromise between what I have and what I should've had.

[image: View Post]JZH, on 09 April 2010 - 06:05 PM, said:

A larger master cylinder gives better feel, at the cost of less power, so you will have to squeeze harder to stop as quickly as you would with a 1/2" master. Many people like this and do it on purpose.

Ciao,

So going from a 5/8" MC to a 1/2" MC I should have more brake power through the lever? I would think pushing less fluid you'd have less? I tried this on my VF and couldn't get much out of a 1/2" MC, it never would firm up (very weak braking power). The 5/8" MC works ok but is very soft after sitting a few days and requires some pumping to get it to firm up. I figured the 5/8" just needed a rebuild, but if what you're saying is correct then maybe it's the calipers that need rebuilding, since the 1/2" MC never would firm up either?

[image: View Post]vfrcapn, on 09 April 2010 - 08:29 PM, said:

[image: View Post]JZH, on 09 April 2010 - 06:05 PM, said:

A larger master cylinder gives better feel, at the cost of less power, so you will have to squeeze harder to stop as quickly as you would with a 1/2" master. Many people like this and do it on purpose.

Ciao,

So going from a 5/8" MC to a 1/2" MC I should have more brake power through the lever? I would think pushing less fluid you'd have less? I tried this on my VF and couldn't get much out of a 1/2" MC, it never would firm up (very weak braking power). The 5/8" MC works ok but is very soft after sitting a few days and requires some pumping to get it to firm up. I figured the 5/8" just needed a rebuild, but if what you're saying is correct then maybe it's the calipers that need rebuilding, since the 1/2" MC never would firm up either?


sounds like you have air in the lines.

[image: View Post]gll429, on 09 April 2010 - 08:37 PM, said:

[image: View Post]vfrcapn, on 09 April 2010 - 08:29 PM, said:

[image: View Post]JZH, on 09 April 2010 - 06:05 PM, said:

A larger master cylinder gives better feel, at the cost of less power, so you will have to squeeze harder to stop as quickly as you would with a 1/2" master. Many people like this and do it on purpose.

Ciao,

So going from a 5/8" MC to a 1/2" MC I should have more brake power through the lever? I would think pushing less fluid you'd have less? I tried this on my VF and couldn't get much out of a 1/2" MC, it never would firm up (very weak braking power). The 5/8" MC works ok but is very soft after sitting a few days and requires some pumping to get it to firm up. I figured the 5/8" just needed a rebuild, but if what you're saying is correct then maybe it's the calipers that need rebuilding, since the 1/2" MC never would firm up either?


sounds like you have air in the lines.

I bled and bled and bled..5/8" MC works well initially, nice and firm, loosens up after a couple days. The 1/2" MC reached a point where it wouldn't firm up any further, the lever reached the bar and that was all it could do.

well then, your lines may be shot..or you may have a tiny leak that allows air in..either a seal or a crush washer..
i have had a the same prblem before.. i put on new lines..and presto.. problem solved..

Hydraulics work backwards from what you would initially think.

Moving a smaller volume of fluid with each stroke (for a given load) gives you more mechanical advantage.

Moving a larger volume of fluid with each stroke (for a given load) gives you less mechanical advantage.

http://science.howst.../hydraulic1.htm

Yes, so going to a smaller m/c gives more "power", but it comes at the expense of more lever travel. And since you only have so much lever travel available, the risk is that the lever will hit the bar before you manage to stop the bike. (Which would be bad...)

Hydraulic performance should not change over a short time--if it does, you have a leak or air in the lines.

Can't access my data table at the moment, but 5/8" m/c is not OEM VFR. All of the 750s were 1/2", as well as the 800Fis, and the VTECs are 14mm.

Ciao,