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This is the front brake MC that came on my single disc '74 Police Eldo. The black Brembo finish was badly faded when I got the bike, so I polished the assembly when I did the first restoration (many moons ago). When I added the second disc to the front wheel, the lever pull got very hard & "wooden". After living with that for several years I broke down & bought a MC from a two disc bike (Ducati I think). I will have the MC pictured for sale at the Mid Ohio swap meet in July.ronkom
Give this a read over:https://www.vintagebrake.com/mastercylinder.htmTom
If you are running the stock single disc DO NOT buy a new or rebuild the stock 15mm master cylinder. It is grossly oversized and is to be used on dual disc setups. Guzzi really fudged this sizing up 40+ years ago. A $16 10mm generic master cylinder off Amazon will give you much better breaking and feel. https://www.amazon.com/Front-Hydraulic-Brake-Master-Cylinder/dp/B0176TAF8YWhile not period correct it'll work 100x better.
I am running dual disks and really want to keep a vintage look above all else. I don’t want to throw a modern looking MC on it.
If you running dual disc you'll be fine with the 15mm MC. MG Cycles has all the parts to rebuild from the reservoir, cap & diaphragm, piston and seal kits.Trust me if you were running single disc you'd quickly get over the "vintage" look for a front brake that actually works.
Single F09 and the 15 mm master cylinder works acceptably well.
It's funny all the different responses you're getting on this. I've found that chart to be worthless. My personal experience has been different.I've ridden the older airheads since 1990 and they've got F08s (when not the earlier swinging ATE) just like these older Tontis.Back in the 70s (and even the 80s) disc brakes were a new thing and the engineers spec'd Masters with bores too large. I've found that three or four steps down was usually best. If a 15 or 17 was spec'd, somewhere around a 12 or 13 worked best. Guys say you've got to have stainless lines for the brakes to work best. I've got both (rubber and SS) and they both feel fine. Perhaps on the track it would make a difference, but on the street it feels the same to me. My 87 LM came to me de-linked. The front master was changed to the larger one customary for dual discs. The old smaller original (a 12mm as I recall) was in the parts box so put it on and the brakes, good before, were even better. So to answer your question, unequivocally I'd say dropping to a 12 would give much better braking. Here's something to think with: Picking the right master is a game of leverage versus lever movement. The farther the lever moves, the greater the leverage and the more feel you'll have. A bore too large will give minimal lever movement and the least leverage. You'll feel like you're pushing on a brick wall. A bore too small, and the lever will come all the way back to the bars without fully clamping the calipers. Some of the airheads came with a 17mm master, and wow - there was hardly any lever movement. Horrible! A 15 is better, but a 13 or 14 made a huge difference. As it is, with the 12mm on the 87 LM, the lever can be pulled half way to the bar, but in actuality the brakes are fully engaged before then. So perhaps an 11 or 10mm master will work. I've never tried. I've heard complaints that the 12 doesn't work - the lever comes back too far and the brakes are mushy. That's not the problem of the master's size, but air in the lines. Been there, done that. That chart comes up every time with this discussion and might be a good rule of thumb, but for sure, real world experience trumps it every time. Listen to those who have been there. Good brake pads, clean discs, calipers that work properly, and the right size master go a long ways to having excellent braking. I've had to stop suddenly a few times and haven't found them lacking.
This is the correct answer. Do you want a bike that looks good on the side stand or stops good while riding? The correct ratio make for two finger braking, good modulation and brake bleeding. Braided s.s. lines are the best choice over rubber. Loosing brake energy thru expanding lines reduces braking power. I have seen a small I.D. master cylinder balloon a rubber brake line.
In 81 BMW finally went with the F08 Brembos which eliminated adjusting and setting of the caliper. So at that time, BMW and Guzzi both used the same brake components, except, as you say, the discs. Yes, the Guzzi cast iron discs have a better friction coefficient and work better. But I wouldn't call the airhead brakes lousy, just the particular bikes you rode most likely needed attention.
The "15 mm" Brembo master cylinders have a bore size closer to 16 mm actual - 15.85 mm IIRC - so yes, very close to 5/8".
That probably means they *are* 5/8 inch, which is 15.875 mm. At least my memory is not failing too badly. The Japanese size their master cylinders in inches too, like wheels.Way too big for most single calipers regardless.