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Ok.
Frivolities over, back to the shed.
My €400 project K1 purchase 6 years ago is now approaching completion. So far it’s only cost me about €1,000 more than if I’d let someone else restore it and then bought it off them.
I’ve got it from no brakes at all to useable stopping via new HEL lines, pads, and a cheap on line master cylinder purchase with smaller bore as so often recommended on this forum.
I’m wondering now if the smaller bore idea works for single disc but not so much for twin disc as the lever has to be pulled most of the way back to the bar to be effective.
Any idea if the original M/C on the K1 is a better bet? I have it but it’ll need new seals at least and maybe the bore will still leak.
Otherwise, any other options recommended, type and/or bore size?
Cheers.

The original master cylinder on my twin disc K packed up 80 mies into a 500 mile round trip.I think this a 19mm cylinder? Engine braking and the rear brake had to do.
I got another m/c which is of a smaller diameter but of the same type. Cannot remember now but think 14mm. I never even bothered to fit new seals.Deplorable!
This has been on for many years and if anything is over braked. I guess this is what riders of modern bikes call two fingered braking. It’s the best brake I have ever had. The lever action is fine and I could never get it all the way back. I do not have a splitter, just two HEL lines direct from the m/c to each calliper.
I would have to check the specifications of the old and new m/c to be certain of my quoted diameters.
John

I think you're right that the smaller bore plan is more relevant to single discs. All the twin disc Morinis I ever tried had perfectly good brakes. My guess is that they hope people who bought the standard single disc bike would upgrade with a second front disc kit and generate some extra profit. If I recall correctly the two cylinders as supplied were 19mm or 16mm. I have a couple of master cylinders lying around redundant even now, having used a smaller Grimeca master cylinder first then a cheapy from a Chinese copy if a YBR 125 Yamaha (a model made in China anyway)
As for the cost of fettling these bikes up, we never learn do we? I'm in the same process right now with four different bikes. I wish I could listen to sound advice from sensible people. I just find sensible people too boring!

Thanks for the replies.
It sounds like either of the original master cylinders may be better than a small aftermarket unit in the case of double discs.
I actually have examples of both so if new seals work it looks like I already have what’s needed.
Like John’s bike my 350K has no splitter, neither did a previous one I had.
I just finished Billy Connolly’s autobiography. He suggests there two kinds of people; those who think motorcycles are dangerous, (the sensible people) and those who think they’re exciting - he prefers the latter. Couldn’t have put it better myself.

Here's some information that might prove helpful when choosing a master cylinder.
http://vintagebrake.com/mastercylinder.htm

Just a word of caution that manufacturers often quote a master cylinder's diameter, but in reality the actual diameter can be a lot different. Don't believe the size until you've measured it

As far as I know all the 350 street models that came originaly with one disc had a 16mm master cylinder. When the two disk model came up with the 500 they changed them to 19mm (I guess the K1 and K2 as well). I think they did it because now there were 4 pistons to be moved. For many years I rode two discs with a 16mm master cilinder and it was ok, allthough it felt some kind of wooden and you had to use some fingerforce.
I have quite short fingers and it came to my mind to try y smaller one because I don´t like to have de pressure point that far from the handlebar where I don´t have that good feeling/sensitivity. A minor master cilinder means a longer way and less force. So I tried a 13 mm mc from Grimeca designed for clipons. The result is that you can pull it to the handlebar with some force without moving the bike. Riding you won´t do that, because you will go down to the ground before! It´s now a two nearly one finger breake, a very significant diference.
I think a 14mm would be the best way to go, because the 13 mm one might be a bit dangerous if you don´t control a fright breaking, especially on a wet road. Or use a 13 mm mc with only one disc also should be quite nice and suficient, and by the way with less weight on the front wheel.

Just looked through my parts I bought for the K1 rebuild.

Caliper pistons are 41mm diameter front and 32mm rear.

Master cylinder seal kits are 16mm and 13mm. The 16mm is the front master cylinder, which came from a 31/2 Sport with a single disc.

All very interesting.
I have a 16mm from a single disc bike so I’ll try that first.
I also have a problem with the double banjo for the Grimeca colliding with the clock (I have the earlier console fitted) but the smaller single banjo clears it so it’s looking like I’ll start off trying 16mm/single disc as I may have no other option anyway unless a 90 degree double banjo exists?

Yes, it is tight. Try looking for something like a Laverda 500 splitter that fits on the bottom yoke.

Picture shows the tightness.

16mm cylinder for twin calipers.

14mm for a single.

If travel is an issue with either of these combinations, then you have a problem elsewhere.

Still quite a heavy lever by modern standards, many will prefer a 12, or 12.7mm (0.5") for a single and perhaps the 14mm for twin.

EVguru wrote: ↑01 Jan 2022 16:47 16mm cylinder for twin calipers.

14mm for a single.

If travel is an issue with either of these combinations, then you have a problem elsewhere.

Still quite a heavy lever by modern standards, many will prefer a 12, or 12.7mm (0.5") for a single and perhaps the 14mm for twin.

I am not sure this is correct. The master cylinder on my K1 was originally on my 1980 350 Sport, cast wheel single disc. The K1 had a Honda master cylinder which went in the bin. This agrees with Norbert's observations below.

The 16mm master cylinder is adequate for the twin disc in my experience. I do not want to be locking wheels, especially in current conditions with water, mud, gravel and diesel on the roads and pheasants jumping out of the hedgerows.

I think the K1 and K2 had different calipers from the earlier bikes. They do appear a lot larger than the earlier Grimeca calipers.

Norbert and I are effectively saying the same thing.

The early bikes use the non-handed Grimeca 1010 caliper, later bikes the 1025. Both use the same 41mm piston and use the same seals.

...but not for the master cylinders.

Norbert says 16mm for single disc, I am saying 16mm for single disc, you have quite clearly stated 14mm for a single caliper.

Am I missing summat?

No, that´s not exactly what we said.

The single disc models had the 16mm mc, in this we all agree. Paul (14mm) and I (13mm) suggested that a smaler diameter would probably be the better choice for a single disc (mantaining the same original caliper)