Choosing correct master cylinder

Jimbonaut

Over 1,000 Posts
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Sweet Jesus I had no idea this was so complicated. I just bought a ⅝" Nissin master cylinder that looked the business and called it good
 

chickenStripCharlie

Been Around the Block
Fot the total picture you nee to consider :

lever length and pivot ratio
mc piston area
total calliper piston area
disk ‘size’ at the effective radius
pad coefficient of friction
wheel radius
tyre coeeficient of friction

way too complex for me ! Hence focus on mc and caliper ratio, and pad grade ( aim for hh or at least gg)

steve
You might be right ... it must be the pivot ratio that makes up for the longer travel that would be required.

Thanks for the information!!
 

chickenStripCharlie

Been Around the Block
Sweet Jesus I had no idea this was so complicated. I just bought a ⅝" Nissin master cylinder that looked the business and called it good
Ya I'm just trying to understand the full theory behind it.

In doing so things get overcomplicated. Most people do what you did and end up fine! Get a good MC that matches the OEM application and call it good
 

chickenStripCharlie

Been Around the Block
Lots of people in my ATV club have changed leaky master cylinders with just random crap they find on ebay with complete disregard for bore size and to be honest, the average joe, after a short adjustment period is none the wiser.

Of course, an ATV that isn't street legal and doesn't see the speeds a bike does is different, but still, people don't really look into it too much.
 

doc_rot

Oh the usual... I bowl, I drive around...
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I think the tire/disk ratio also plays a large part as well. If you have big brakes on a small diameter tire the rotor will have more mechanical advantage than a small rotor with a big diameter tire. also the mass of the wheel has a significant effect as well.

My guess as to why manufacturers stray from the ideal ratio is for the reasons mentioned above and that the OEM is mixing and matching components across their product line. Sometimes they settle for less than ideal setup to save some money by hitting larger quantities on parts.
 
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chickenStripCharlie

Been Around the Block
So, what is master cylinder pressure supposed to be like?

I can't get my brakes to bleed at all. Been at it for over an hour and i can't build up any decent pressure in them.
I took off the lines at the MC and plugged with my finger. I gave the handle a few pumps and could feel a bit of pressure under my finger, and when i take my finger off i hear a puff of air come out. That ought to be enough, right? I did notice that not a lot of fluid came out, but i'm not sure how much is supposed to come out with each squeeze.

I have got fluid in the lines to a point where fluid comes out of the bleeder valves, but it comes out quite slowly. I would expect that by now it should shoot out a little quicker than that, with the lever held in and when the bleeder is opened.

The lever is still easily pulled to the bars just with my pinky finger. I also would have expected to have gone through more brake fluid by now than i have. As there is no pressure built up at the lever, there is nothing to really force the brake oil down, i guess?

I even left the limp lever tied to the grip overnight and when i went to check this morning, i didn't notice that the oil level in the cup had dropped at all, so i don't suppose any air came up.

I was getting bubbles to start, both at the MC and caliper. Now, i don't get bubbles, just fluid comes out, but i don't get any pressure either.

I can't figure it out. Any ideas please? :)
 
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ridesolo

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Don't know if this will help you, disregard if it's of no value. When I went to bleed the brakes on the Hedgehog I wasn't sure how it was going to go. Did the rear first, aftermarket reservoir, Yamaha R-1 master, and Kawasaki caliper. pumpity pump, pumpity pump, bleed bleed, and pretty soon everything was perfect. On the front it is another aftermarket reservoir, an aftermarket (probably ChinaBay) master, and Kawasaki caliper. pumpity pump, pumpity pump, no bleed. repeat, repeat, repeat, and on and on and on. Nothing. Finally did a little research and learned that there are two little holes in the master, an Intake Port and the Compensating Port (or something like that). I took the master apart and discovered that at the factory they had drilled the hole for the Intake Port but the hole for the Compensating Port hadn't been drilled all the way through. I checked the internet image again to make sure I wasn't being a dufus and then used a pin vice and a very tiny drill and completed that second hole. I made sure everything was cleaned out, put it all back together, and pumpity pump, pumpity pump, bleed bleed, and everything was perfect!

Like you, before I drilled out that little hole I had very little fluid and little to no pressure. Might be worth taking that master apart and check things out.
 

chickenStripCharlie

Been Around the Block
Don't know if this will help you, disregard if it's of no value. When I went to bleed the brakes on the Hedgehog I wasn't sure how it was going to go. Did the rear first, aftermarket reservoir, Yamaha R-1 master, and Kawasaki caliper. pumpity pump, pumpity pump, bleed bleed, and pretty soon everything was perfect. On the front it is another aftermarket reservoir, an aftermarket (probably ChinaBay) master, and Kawasaki caliper. pumpity pump, pumpity pump, no bleed. repeat, repeat, repeat, and on and on and on. Nothing. Finally did a little research and learned that there are two little holes in the master, an Intake Port and the Compensating Port (or something like that). I took the master apart and discovered that at the factory they had drilled the hole for the Intake Port but the hole for the Compensating Port hadn't been drilled all the way through. I checked the internet image again to make sure I wasn't being a dufus and then used a pin vice and a very tiny drill and completed that second hole. I made sure everything was cleaned out, put it all back together, and pumpity pump, pumpity pump, bleed bleed, and everything was perfect!

Like you, before I drilled out that little hole I had very little fluid and little to no pressure. Might be worth taking that master apart and check things out.
When i rebuilt the master i made sure this little hole was there and clear! I saw some bubbles come out of it at the start of my bleed process. It's gotta be something else.
 

Maritime

Over 10,000 Posts
I've had similar issues, what I found to be best is to use a large syringe and to open the bleed screw at the caliper and force fluid from there up to the bars and it sends all the air out along with it. I've never really had luck starting from the master and pumping fluid from there to get a system full and working. After I have it all filled with the syringe and time comes to change fluid I can open the bleeders and pump old out and fill new and it all works but not when starting fresh with an empty system.
 

Jimbonaut

Over 1,000 Posts
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My neighbour just bled his Nighthawk brakes the same way. Borrowed a syringe from me and filled the system from the caliper not the the mc.

With a dual disc set up do you need to crimp off one caliper line while filling the other? Guess you can't crimp a braided line - block it off somehow? Or is this not necessary?
 

Maritime

Over 10,000 Posts
I just did one till fluid got to the mc then closed that bleeder and did the other. Put the cap on the MC or a rag as it will rain brake fluid when you get the pressure built, ask me how I know
 

Jimbonaut

Over 1,000 Posts
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Ha, messy. Duly noted. Question - once you'd filled the lines from the calipers, do you then bleed the system using the brake lever, cracking one bleed valve then the other?
 

chickenStripCharlie

Been Around the Block
Thank you guys, i did look up on youtube how to do reverse-bleed, but i thought it was not necessary considering i was starting to get fluid down at the caliper bleed screw.

When doing it in reverse .... do i keep the lever tied to the grips? or lever let out?
Do i leave the bleeder screw on both calipers open, or just the caliper i'm injecting fluid at?

Thanks again!
 

Maritime

Over 10,000 Posts
open just the bleed you are working on. I left lever in the out position. I have the double banjo so I did the caliper with the line closest to the MC first, then did the other.
 

chickenStripCharlie

Been Around the Block
open just the bleed you are working on. I left lever in the out position. I have the double banjo so I did the caliper with the line closest to the MC first, then did the other.
I did this and I have brakes!!!!! Thank you, sir.

Went and got a syringe, did the line furthest from the MC first and got A LOT of bubbles up at the MC. I went slow and did not make a mess at the MC at all. Once that was done the other side didnt even need to be done. Took care of bubbles in both.

Did a conventional bleed on both of them again after and nice clear fluid came out sans bubbles.

Good pressure. Good lever feel. I like it!

Thanks a bunch guys! I watched some YouTube vids on this before but I just assumed since I was getting fluid at the bleeder that problem was somewhere else.
 

Maritime

Over 10,000 Posts
glad it worked. Like I said. I've never been able to get a multi caliper system bled from the mc. A single seems to be fine but the double I always have to do with the syringe
 

chickenStripCharlie

Been Around the Block
glad it worked. Like I said. I've never been able to get a multi caliper system bled from the mc. A single seems to be fine but the double I always have to do with the syringe
This was my first dual caliper bleed!

As soon as I started with the syringe, I could hear the bubbles fizzing up at the MC. Boy, was I glad that's all it was.

Still curious why the other way doesnt work. I suppose just not enough piston travel in the MC to squeeze enough and accomplish the same as a syringe
 

Maritime

Over 10,000 Posts
the amount of fluid the mc moves innone pump is very little. Just to fill the lines takes 100's of pumps then you need to try and get bubbles out. The syringe puts a lot of fluid in each time.
 

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