I haven't been able to find that info anywhere. It's a 4 piston but I don't know what size they are. Maybe someone will chime in, they're 2017 calipers from an FXDLS (they're going to be for sale soon too)

 

If I'm doing my math right.....
8 ea 30mm pistons (4 per caliper)

11/16" (about 17mm) would be a ratio of about 23:1. That "should" make it more of a wooden, shorter travel, more of a on and off feel.
5/8" or 16mm would be about 28:1. That should be a bit on the soft longer travel side easier for 2 fingers.

Hope this helps,
Tom

 

I can't tell if it's wooden or not since I can't ride it, but I can say it doesn't feel like it will be. The main issue is being spongy, and the secondary issue is too much travel. I hate math so I couldn't tell you if you're correct or not, the document I attached in the first post (Front Master Cylinder Ratio Chart (vintagebrake.com)) is pretty different than your calculations...who knows how accurate IT is though? The more I dig on the internets though, the more I see 19mm masters used with dual Brembo radial calipers and 15 or 17mm used with a single.

 

I used the 30mm 4 piston spec and doubled it. The listing I believe is for a single caliper, not dual.

Can you pull the lever to the grip? Can you pump the lever to get a firm feel? Then it's likely air especially if pumping helps.

If it has a lot of travel, then will go to a firm feel, likely too small a master. Your choices per the chart a pretty limited.

FWIW: This is the sizes from one of my bikes. Front wheel 18", rear 17". Brakes are Brembo.

Front 320mm disc, single 4 piston caliper (2ea. 30/34). master is 13mm. Lever moves a little then controlably gets firmer.
Rear is linked to a second front caliper. Rear disc is 282mm with a 2 piston (2ea. 30/32) caliper, front disc is a 320mm with a 4 piston (2ea. 30/34) caliper. Both are being fed by a single 16mm master.

Hope this helps,
Tom

 

Ah, gotcha. And thanks for the info on your bike, very helpful!

 

I should have done this earlier. My front should have a 25:1 ratio if my math is correct and the chart is correct. They don't list 34mm pistons, went with 35mm. If I used 32mm it was 22:1. So just a guestimate, my 30/"34"mm is likely 23.5. It's not really a progressive pull, but not wooden either. Remember this is a single caliper (one side of wheel) with 4 pistons, not a dual 4 piston caliper (both sides of wheel) set up. So 25:1 would be right about the middle of what the chart guy said, guess the bike maker got it about right per the chart guy.

Area-mm2 of a pair of 30mm is 1413, area-mm2 for a pair of 35mm is 1924, MC 13mm is 132. 1413+1924=3337, then divided by 132=25:1 ratio. This is round numbers, didn't use behind the decimal point. Again if my math is correct for what the chart tells me. First time I have worked out the ratios.

Tom

 

If the lever has a lot of travel, and the master is the issue, then a bigger one will decrease travel. There is nothing you have mentioned that would indicate a smaller master will help.

 

Yeah, I was going the wrong direction earlier in the thread, I had it in my mind that 15mm was larger than my 11/16". I may try and reverse bleed since I have the time, but I'm convinced it's the master size and not an issue of air.

 

You can also measure the diameter of the pistons in your original brake and get a base line. Dial calipers work best, but a ruler will do in a pinch.

 

The ratios you are calculating are really 1/2 that as opposing pistons don't apply to the pressure of the pad. It's 4 30mm pistons not 8. The opposing side only adds to pad travel.