Disc brake master cylinder size
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
I extracted this information Greg Field, Rob Anderson, Steve Odell, and Charlie Mullendore of Antietam Classic Cycle off of the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). Vintage Brake is another good source for disc brake ratio information.
In Greg Field's own words:
Brake master cylinder-caliper ratios are all trade-offs. If you want more power, go to a smaller master cylinder. The trade-off for the power is longer lever travel. I don't mind lever travel, so long as I can lock the brake before the lever hits the hand grip. Given that, here's what I think is ideal:
- For a single F08, and 11 mm master cylinder is perfect. A 12 mm master cylinder is nearly perfect. A 13 mm master cylinder works pretty good.
- For dual F08s, and 13 mm master cylinder is perfect. A 15 mm master cylinder works pretty well.
Here's what I advise customers who call my shop:
- If you think you will be keeping it single-disc, get an 11 mm or 12 mm master cylinder. The 11 mm master cylinders in big-twin sizes (most are tiny ones with short levers for dirt bikes) can be tough to find.
- If you are sticking with single-disc for now but might want to upgrade to doubles later, get a 13 mm master cylinder. The 13 mm works pretty well with a single F08 and works stunningly well with twin F08s.
In Rob Anderson's own words:
Guzzi use 13 mm front master cylinder as std for the Cali where is actuates a single 4 pot Brembo caliper onto a 320 mm disk. I am using a 5⁄8 inch (~16 mm) master cylinder to operate twin 4 pot Brembos (with asymmetrical piston sizes). In practice the 5⁄8 inch front master cylinder is pretty handy for dual F08 calipers as well and easy to get hold of from a wrecker, CB900 with integrated reservoir if I remember correctly.
In Steve Odell's own words:
16 mm master cylinder makes the squeeze no different than a 12 mm - 13 mm for a single disk. I have tried 12 mm, 13 mm, 15 mm and 16 mm. 16 mm was best. Use stainless braided lines. Using rubber lines is a worthless waste of pressure. And get rid of that brake switch that is normally on the lower tree T junction block. That's another waste of pressure and a place where air bubbles get trapped and you never can get them completely out.
In Charlie Mullendore's own words:
The 13 mm master cylinder that I swapped Tom the 15 mm one for made a huge difference in the stopping power of my customer's Eldo. With the 15 mm m/c it was like grabbing a hand full of rock and the bike barely slowed at all. Stainless lines and EBC HH pads made very little difference.
With the 13 mm m/c it now stops very well without being too grabby and has nice feel. This is how disc Eldos should have been built in the first place! No issue with bleeding the system even with the stock switch in place - the lever is nice and firm. Neil is going to be very happy!