what master cylinder piston size for 40mm ATEs?

[DucatiPete]

I'm upgrading my '79 RSs front brakes, which includes adding a later model handle bar mounted master cylinder. I've rebuilt the 40mm ATE callipers - new pistons & seals. And added SS lines.
The m/c I've got is a 13mm bar mounted magura. Now, after bleeding, the lever pulls all the way to the bar - grrr - not good, in fact totally unacceptable.
The original under tank M/C has 17 cast in it's body. I assume that means it's got a 17mm piston. If that's the case, I'm guessing I should use a similar sized bar-mounted unit. I see 16mm units are available.
Any opinions as to if a 16 will do the job - and possibly offer reasonable brake lever 'feel'.
The older I get the more I appreciate reasonable braking.
Thanks in advance.
Pete

[ME 109]

Welcome back Pete!

Having poor brakes is what helps me whip sports bikes around corners.

Someone will have done just what you're doing......... they'll chime in soon.

[SteveD]

Pete, try filling the reservoir from the bleed nipple up. It can make a difference to the initial bleed.
You'll need a syringe and some tubing. Do you want me to send you a syringe?

[DucatiPete]

I'm pretty sure they're bleed ok, as the lever didn't pull all the way in and felt pretty good when i had the callipers chocked with packers. The packers were slightly thicker than the disks. I reckon not enough fluid is getting pushed...

[Seth]

I have a 77RS with the under tank master from a single disc bike, which I believe is 13mm. The level is soft, and with a lot of effort I can probably pull it back to near the bar, but in daily riding I never need to go that far. I don't believe it's as good as a handlebar mounted master (cable friction and stretch) and not sure about the mechanical leverage difference (at both the lever on the bar and the lever on the master), but in the end 13mm is 13mm and the same travel will create the same pressure at the calipers. I'd try bleeding again, as often a small bubble can be caught in the end of the master or any horizontal line (you probably have less of these because of the bar mounted master).

[Jean]

Keep bleeding. Seth's right: There's AIR somewhere, or the MC you mounted is faulty.

[chasbmw]

13mm is fine on my 38mm ATEs, try tying the brake lever back and leave it overnight, if you are lucky the brake bleeding hobgoblins will visit you and sort it out. If you go back to 17mm you will end up with wooden brakes and the requirement to sizexxxl hands to get much effect. BMW in common with Nortons and maybe others were very conservative with their brake ratios with the early disc brakes. Too afraid of people locking the brakes and locking the fairly crappy tyres that were available in the 70s.

At the end of the day lever travel is much increased, but this is what enables you to modulate the front brakes properly and when I tested my brakes by doing a series of crash stops from about 70mph, there was no danger of the brake lever coming right back to the bars.

[Mal S7]

Hi Pete,
I am interested in what this thread turns up. As I told you, my handlebar mounted magura m/c from Motobins is 14mm and that works pretty good with a single disc and 40mm ATE caliper. I am encouraged by Chas and others that I should be able to keep using it when I change over to twin calipers this Summer. I might be doing stoppies at the traffic lights but!
cheers
Mal

[DucatiPete]

I'll revisit the bleeding, but i reckon I'll need a larger dia m/c... If, apparently, the 13mm m/c is ok with 38s I suspect it's not quite big enough for 40s... Therefore, a 15 or 16 should be better, I reckon.

[chasbmw]

Pete, just stick with what you have got, it will work, there are other people using 13s on 40s, it will work, just get it bled then try it out before you change.