master cylinder question

[jackstraw650]

I've just gathered the parts to upgrade my '79 standard to a dual disc set up. After reading as many threads as I could find, the general consensus of opinion seems to be that the stock 14mm M/C will work OK but that going to an 11 or 12mm will improve braking performance and "feel". The problem that I'm having is finding which M/Cs have an 11 or 12mm bore. Very few of the ads on EBay give that info so I'm not sure which one to buy. Does anybody have a recommendation or a list of which M/Cs have what piston size? Thanks, everyone!

 

[grizld1]

The master cylinder piston area is selected based on total caliper piston area. The 11 mm. MC is a good choice if you're using a single OEM floating caliper, but you'll need a MC with a larger piston when you add the second caliper to the system. Have a look at Michael Morse's chart at www.vintagebrake.com .

 

[jackstraw650]

Thanks for the reply, Griz....but that's where I checked the figure of 11-12mm. I think it was a post by XSLeo[I think, but don't shoot me if I'm wrong, Leo!] that I found in a search that I saw the figure. Maybe I'm reading the chart wrong or measuring the caliper piston wrong but I measure the caliper piston on the stock caliper at close to 40-41mm. Then I go across the top of the chart at 11-12mm and find at the 41mm spot ratios of 13.89 to 11.67 which are in line with his recommendation of 12-14 with single piston calipers. This is where I get puzzled: if there are 2 calipers as with a dual set up do you use the figures for two pistons in parentheses? But if you do you end up with the same 11-12mm M/C to put you in the recommended 27-1 ratio area........or have just totally fracked up the whole thing??????
Thanks for any help in spite of my ignorance!!

 

[grizld1]

Remember, you're using two single-piston calipers--you don't go to 27:1 with those, see Michael's lower ratio recommendation for single-piston brakes in the last paragraph! A 16 mm. MC from a 72-76 model would be in range for you.

 

[jackstraw650]

Griz, I realize that, see my post above where it starts..."Then I go across the top of the chart at 11-12mm" The whole key to this as far as I can see is what diameter are the caliper pistons in '79 single piston calipers? I seem to remember reading somewhere that the 72-76 M/Cs were a good choice. Thanks for the help, Griz!

 

[XSLeo]

I have a dual disc brake set up. I have a 16 mm M/C, using it I now better understand the what wooden means. The brakes work well. It takes a fair amount of effort to pull the lever. When you pull the lever they feel very firm and the lever travel is very short.
I have a 14 mm M/C that I tried. This M/C workes better. The lever pulls a bit easier. The travel is about twice as much. This allows me to feel the brake action better.
The 16 mm M/C, I don't feel I have as much control of the brake. If the brake starts to lock up I can't feel it because of the extra effort to pull the lever.
The 14 mm M/C I can feel when the brake locks better and I can modulate it better.
This 14 mm M/C leaks so I need to rebuild or replace it.
The 11 mm M/C works fine with a single piston caliper on a single disc set up.
The 14 MM works well with the two single piston dual disc set up.
On the older two piston caliper single disc a 14 mm should work ok. Going to a dual the 16 mm should work.

 

[jackstraw650]

XS Leo, thanks for your examples! I understand exactly what you are describing....I would rather have the longer travel and better modulation any day. I have a '79 Standard and its my understanding that it has a 14mm M/C stock. If that is true then I should just stick with it for my new dual single piston caliper set up? That would be great.....one less thing to spend money on!
Thanks again, Leo!