what master cylinder piston size for 40mm ATEs?

[tagordon]

Reply to Jean
The belief that Bumble Bees should not be able to fly is based on fixed wing aerodynamics.
Using aerodynamic principles that apply to a rotory wing such as a helicopter, then it is clear how the Buble Bee is able to fly.
Then there's the Magic Dust for Santa and his reindeer.

[DucatiPete]

OK, I've just got home after being away for the w/end.

Thanks very much for the various learned comments - truly one of the greatest things about this forum!

Since getting home I've just had enough time to tape the lever back to the bar, as it'll be a couple of days before I'll get a chance to have another go at bleeding...

However, I remain dubious... I've never had problems bleeding ANY brake system such as the way these things are (not) performing.

I keep thinking: How come the lever pulled up pretty well when I used packers (slightly thicker than the disks) between the pads? Although it was still pretty inadequate for a useful/safe braking system.

One strong variable is that the m/c is secondhand, therefore subject to being damaged/worn... I remain optimistic that it isn't.

It does appear, from the thread's various comments, that a 13mm diameter m/c piston should indeed push enough fluid to lock the wheel before the lever touches the bar... In fact even a 11mm m/c would appear to be adequate.

I'm guessing that someone has *personal* experience of having a setup like mine WHICH WORKS - ie they have a bike with twin 40mm ATEs being properly operated by a 13mm handlebar mounted magura m/c?

I was wondering if someone could put up their hand and provide an example of this setup actually working - as opposed to the theory that it should?

[chasbmw]

Pete,

it really does sound as if you don't believe us.................about brake bleeding, last weekend i changed the handlebars on my R90/6 with a 13MM BMW handlebar MC. To do this I changed the upper brake line, I had to bled the brakes, I had gone through about 3 MCs worth of fluid, and the brake was still spongy, i tied the lever back to the handle bar and left it overnight. [During the night the brake bleeding hobgoblins visited my garage and spent all night fixing the brakes] and in the morning the front brakes were fine, with no further work needed.
This does presume that there are no leaks in your system

I cant specifically talk about aftermarket Magura MCs, but 13MM is 13MM and I doubt there would be much mechanical difference between the 13mm Magura brakes fitted to the late 80s airheads and your aftermarket Magura lever. in terms of the ratios 40/17=2.35 40/13=3.08 38/13=2.92. I don't know if you have ever used the stock brakes, but they are pretty awful and I can assure you that the revised set up is much nicer to use, on my bike the lever will come back to within 1/2" or so of the lever, but not further even under provocation.

[jagarra]

Another great thread. I too have used the method of tying the lever overnight and it has made a big difference in the bleeding of the brakes, nice and solid the next morning. I am going to include a link to the brake ratio/mc site again as this has some very interesting information and things to consider when making changes to the brake system. A point to remember is that the active pistons will change the ratios, our ATE calipers only have 1 active piston. I have a 13 mm MC with a 40mm caliper, but I may use 2 38mm calipers when I go to dual disks as it lowers the ratio, but I end up closer to the 15 to 1 ratio the site recommends for 1 active piston calipers.

http://www.vintagebrake.com/mastercylinder.htm

[Jean]

Firm believer in magic dust for all things.
Including bleeding brakes.

However, Duc-pete, the statement that the mc is second hand STILL leaves the distinct possibility that IT is the gremlin.
Do the best job you can bleeding the brakes the "normal" way, then do the tied-lever thing.
After an overnight or however longer than that you wait till you get back to the bike...if the brakes aren't bled OK, you have another problem.
BTW, a really BIG mc will make the EFFORT to apply the brakes tremendous.

[Roy Gavin]

When I converted to twin disks/ handlebar MC I used a secondhand perch ,13 mm MC and lever.

At least it looked like a 13mm, but the brake was terrible- week and long travel.. On checking I found that someone had overhauled the MC by boring it out to 16 mm, and fitting a larger piston and seal, so I replaced it with a new 11mm MC and this transformed the braking power. There was a fair bit of lever movement before it started compressing the fluid, but a lever with a longer nose cured that, so I assume that I had a whole bunch of mismatched bits.

That chart with cylinder ratios of 15 for single piston swing callipers may be correct for lever travel, but I don't think it is correct for power, and even if it was it still ignores the fact that a swing calliper just isn't as efficient as a parallel pair of pistons.

[Jean]

Roy, that comment aboout a "fair bit of lever movement before it started COMPRESSING the fluid implies there was some air in there; that the brake lines were expanding (Old); or the various clearances between the lever, mc cyl, etc weren'tt right.
OH, and a note to all. In the 2003 CLYMER where it discusses the under tank mc, I didn't see any mention of the need to have clearance between the tit on the lever and the end of the piston. I have a little gauge I made from a washer, filed to the correct thickness to do this. Supposedly a gauge for this CAME in the original tool set for all those bikes with such an mc set-up.
Just a note to the wise.
If it's not done, your brakes will drag, and likely lock-up at the least desirable time.

[Motorhead]

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

what method did you do

I found over nite gravity bleeding the best in my BMW system

make sure the resevoir is full bungiee the lever back and let sit usully a top up and 1 to 2 pumps with my early 13mm under tank full power at 1/2 travel to the grip highest lever on a beemer I ever saw

maybe the Honda piston I adapted is the reason there

[DucatiPete]

OK, a quick update...

I've had the brake lever 'under bungie' for a few days. The lever pull resistance has definitly improved. Now the brake lever pulls back to about 1/2 inch from the bars. However, if i use a little more uumph I can 'force' it back to the bar - it's still a little squishy. But it's prolly worth a proper test. I'll keep playing with it (ie more bleeding), as it'll be a little while before i get to ride it under road-braking conditions.

I'd like to try a 14mm m/c for comparison, but will keep futzing 'coz I understand this combination should work.

Thanks again for your unswerving help & perserverance.

pete

[DucatiPete]

full power at 1/2 travel to the grip highest lever on a beemer I ever saw

That sounds pretty 'squishy' to me, but I'm more used to the Magura/Brembo combination of later airheads - ie, more like squeezing a brick, but plenty of stopping power.