Fr Master Cyl, bleeding

Joined
Jul 10, 2004
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I've fitted a new Magura front master cylinder from Rocky Point Cycle, along wiht a Spiegler braided line, custom made length for use with my clip-ons (no going back!). This is a nice piece and uses a small mechanical brake light switch, very clean.

I had recently bled my front brake (with original cyl), enough to replace most of the fluid, which was my intention. Got all the bubbles out in 2 minutes and resulted in a very solid feel, as if it was hitting a block of wood. But it was weeping slightly and I wanted an improved feel.

After installing the Magura and the new line, I bled very carefully, probably went thru the entire reservoir twice, being careful to keep it topped up. But the lever is still very easy to pull, all the way til it hits the twist grip. I know the leverage is quite different, but it should stop somewhere? What am I doing wrong? I thought I knew how to bleed brakes, particularly with a single 19" line down to the caliper. Is there another bleed fitting I don't see? BTW - the brake IS functional, stops the wheel, but I have NOT been out on the road yet.

Stuart Ostroff
 
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Jul 10, 2004
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Stuart:

I just checked my Magura master cylinder. the lever closes to about an inch from the grip.

Compared to the stock m/c, my Magura feels mushy but it works okay.
 
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Dec 19, 2004
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Hi Stuart,
You may have to lean the bike, turn the bars, reposition or even remove the master cylinder from the bar in order to get it in a flat position (IE eliminate any uphill potential air pockets).

justa thought,
 
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Apr 15, 2004
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I believe that Magura has an 11mm piston meaning longer lever travel than most but it shouldn't be hitting the grip. Sounds like you still have some air trapped in it. Did you get the weeping fixed? That could be sucking air back in.

One trick someone told me is to hold the brake lever in with a rubber band or something and let it sit like that overnight. Supposed to help let the air work its way to the top.

I just put a Nissin mc from a CBR600 on my bike. Seems to work very well. I'm waiting for the weather to improve (snowing today) so I can take it for a longer test ride.

Debby
 

TT

Joined
Mar 7, 2005
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53
Stuart -I have the Grimeca 13mm M/C and had the same problem.

I found an adjustment screw at the base of the level. By screwing it in you activate the M/C earlier - essentially taking out the slack in the mechanism.

The only thing about the new setup that is better is the Magura mechanical switch. It still stops like bus ...
 
Joined
Aug 11, 2003
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I too fitted a Magura master cylinder and went through a complete bottle of fluid trying to get rid of the air. Just kept filling and bleeding without really any sign of air coming out but the lever went all the way to the grip. Then it just seemed to work perfect. Just have to keep trying.

Jim
 
Joined
Jun 14, 2003
Messages
747
Sometimes air becomes trapped at the master cylinder. In this instances, try "cracking" the banjo fitting at the master cylinder for a moment while depressing the brake lever.

Jason
 
Joined
Dec 24, 2003
Messages
362
Stuart

I had a similar problem fitting a brembo master cylinder.
It didn't seem to matter whether I "cracked" the line where it leaves the master cylinder or just bled it normally.
I tried everything it seemed.
Even tried leaving the caliper bleed screw just open a little for a few hours to try letting it bleed itself.

Then when I left it overnight & applied the brake, it worked !!

Having had two bikes but only one master cylinder for a while now, I have done a bit of swapping with the brembo master cylinder. Always had the same problem you seem to be having.

Today though, I put an original norton lockheed master cylinder together & installed it with the standard caliper & it bled easily.

Aren't the norton lockheed master cylinders a bit of fun........
Trying to get that lil cup into the passage then holding the piston in while trying to insert the lever & bolt :!:
 
Joined
Jul 10, 2004
Messages
69
UPDATE - Success!

Thanks for all your messages - this forum is so helpful!

After another very careful bleeding attempt and a ride yesterday with less than adequate front brake, I tried Debby's suggestion. I bungeed the break lever with moderate pressure on it and let it sit overnight. This morning I went out to the garage, held my breath while pulling off the bungee, and presto! The lever has a really solid but gradual feel, and stops about 1-1/4" from the grip. There must have been some air somewhere which worked its way out. I had also adjusted the screw at rear of lever and took a little more slack out of the movement, and will probably leave it there.

Can't wait to take it out tomorrow for a nice ride! Thanks again!

Stuart :D
 
Joined
Jul 10, 2004
Messages
69
Follow up from Stan

I sent my "Update" post to Stan at Rocky Point Cycle Supply, who supplied my Magura master cylinder and brake line.

This is his reply:

That's a good trick. The master cylinder with the smaller piston pushes less fluid, and is a bit harder to bleed, especially since it is trying to push the air down the brake hose when it wants to go up! After your ride, try bleeding again; the bumping around makes the bubbles come to the top of the line and sometimes they "burp" out upon bleeding.

Stan
:idea:
 
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Jun 14, 2003
Messages
747
Just for the record, a smaller master cylinder will “push” the same amount of fluid as a larger one, provided it is being used with the same caliper. However, a smaller master cylinder piston means more lever travel.

Jason
 
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Dec 24, 2003
Messages
362
Jason said........

"Just for the record, a smaller master cylinder will “push” the same amount of fluid as a larger one, provided it is being used with the same caliper. However, a smaller master cylinder piston means more lever travel."

Well, "Just for the record", how do you explain why the brembo master cylinder lever only has to be pulled in about a fifth of my norton lockheed lever when used with a norton caliper ?

The brembo has a 12.7mm bore while the lockheed has a 16mm bore.
The brembo certainly doesn't travel anywhere near as far as the lockheed.
 
Joined
Jun 14, 2003
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Nortonfan,

Allow me to explain:

The piston in the disc brake caliper must travel a certain distance in order to push the brake pad against the disc rotor. Now, let’s assume this distance is 1/16”. So, if the diameter of the caliper piston is 2”, the swept volume can be easily calculated. In this theoretical case the volume is 0.2 cubic inches.

Regardless of the size master cylinder being used, the caliper piston must travel 1/16” in order to push the pad against the rotor. And it takes 0.2 cubic inches of fluid to do so.

So, a master cylinder with a small piston must travel farther than one with a large piston in order to displace the same 0.2 cubic inches of fluid. Therefore, given the same brake-lever geometry, the lever travel will be longer on the master cylinder with the smaller piston.

Feel free to contact me if you're still confused.

Jason
 
Joined
Dec 24, 2003
Messages
362
Wow, that is so amazing Jason.

Thanks for that explanation. We should have no problem bleeding our master cylinders with that information on display.

That also explains why my brembo master cylinder piston is so much shorter than then lockheed norton one. It is because the piston has to travel so much further.

It is clear as mud.

The only part that confuses me is why you have to complicate simple issues for people.
 
Joined
Jul 10, 2004
Messages
69
Hold on guys!

I didn't mean to start something here - I see drama in these words. And I don't know who is being sarcastic.

If the Magura (or Brembo) has a smaller diameter piston it must move further in order to displace the same amount of fluid to move the caliper piston a given distance. And that difference is the inverse of the square of the radius. The original master cylinder has a larger piston, moves LESS distance, but has less "feel". Using a smaller piston will improve the feel and control.

I had a problem, this forum provided a clever solution, now it is fixed. DONE.

Thanks, Stuart Ostroff
 
Joined
Apr 13, 2005
Messages
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Jason: just to state the obvious about this thread, for any new out there,by doing all this work we increase the lever to piston ratio. This gives us much more pad pressure for the same lever pressure.

Bill E. :) :)
 
Joined
Dec 24, 2003
Messages
362
Fixing other peoples problems is the whole idea of contributing to this forum Stuart. You haven't started anything.

Glad to hear that your problem is fixed :D

When people make statements that are incorrect/false, I like to discuss it.
Jason made a statement that I know to be incorrect through first hand experience.

He said "However, a smaller master cylinder piston means more lever travel."

That doesn't happen with my brembo setup, in fact the lever only comes in about a quarter of the distance my lockheed does with the same setup.

None of this info actually helps anyone to bleed thier master cylinder & is useless information.

So, like you say let's move on.
 
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