Topic: I need a new master

[ronlarimer]

Before you suggest it, I searched and read through the FAQs

I am doing the dual disk mod to my 73 cb750 and I know that will require a new master cylinder.  I can not find any information on what size master people are using successfully, or ever what size the stock caliper piston is so that I can use a reference chart.

The closest thing to logic I have found was "Bike A" had 2 calipers so I used it.  Not considering the number of pots or their size.

Thanks.

[sandcastcb750]

A stock master cylinder is sufficient. Once the pipes are filled, very little fluid is displaced during braking.

[BobbyR]

I just rebuilt my MC using all OEM Honda Parts and it made a big difference. With SS lines she stops really well. It cost me like $60 for everything.

[batdan]

Well stock master cylinder size is 14mm and the cb750k4 caliper piston diameter is 38mm (I just went and measured my old one just for you)

The hydraulic ratio being the important thing so

All Stock
38^2/14^2 = 7.37

Stock master cylinder and dual front calipers.
(2*38^2)/14^2 = 14.73

16mm master cylinder and dual calipers
(2*38^2)/16^2 = 11.28

From the chart at http://www.vintagebrake.com/mastercylinder.htm it says that single piston calipers like a ratio of 14:1 to 12:1. The stock ratio is well below this meaning that you have to pull the lever harder but not as far to get the same braking force. A larger ratio means that you have more lever travel and less effort to brake. You probably want something from 14mm to 16mm. Personal preference maybe. Pads and lines make a difference too.

I suspect one of the biggest reasons people notice braking improvement when going to dual disks is the master cylinder ratio changes.

These guys have all the sizes of Brembo master cylinders you could need plus they also sell things like aluminum banjo bolts and custom braided lines at exorbitant prices. The regular master cylinders seem to be priced pretty reasonably though. It's where I bought my stuff.
http://www.yoyodyneti.com/Category.aspx?CategoryID=3006

*Disclaimer: I ignored Hondaman's advice recently when I purchased an 11mm master cylinder for my single disk setup giving me a ratio of about 12:1. I haven't really ridden it yet so hopefully it doesn't kill me. I haven't actually done anything involving dual front calipers.

[TwoTired]

Very useful post batdan.  Thanks for the effort and the references!

Cheers,

[ronlarimer]

Well stock master cylinder size is 14mm and the cb750k4 caliper piston diameter is 38mm (I just went and measured my old one just for you)

The hydraulic ratio being the important thing so

All Stock
38^2/14^2 = 7.37

Stock master cylinder and dual front calipers.
(2*38^2)/14^2 = 14.73

16mm master cylinder and dual calipers
(2*38^2)/16^2 = 11.28

From the chart at http://www.vintagebrake.com/mastercylinder.htm it says that single piston calipers like a ratio of 14:1 to 12:1. The stock ratio is well below this meaning that you have to pull the lever harder but not as far to get the same braking force. A larger ratio means that you have more lever travel and less effort to brake. You probably want something from 14mm to 16mm. Personal preference maybe. Pads and lines make a difference too.

I suspect one of the biggest reasons people notice braking improvement when going to dual disks is the master cylinder ratio changes.

These guys have all the sizes of Brembo master cylinders you could need plus they also sell things like aluminum banjo bolts and custom braided lines at exorbitant prices. The regular master cylinders seem to be priced pretty reasonably though. It's where I bought my stuff.
http://www.yoyodyneti.com/Category.aspx?CategoryID=3006

*Disclaimer: I ignored Hondaman's advice recently when I purchased an 11mm master cylinder for my single disk setup giving me a ratio of about 12:1. I haven't really ridden it yet so hopefully it doesn't kill me. I haven't actually done anything involving dual front calipers.

This is exactly what I was trying to do.  Thanks!

[sandcastcb750]

If your brake lines expand during braking, they are moving fluid but at a pressure loss.

The suggestion for high performance brake lines is a good suggestion.

If the system is good, the piston hardly moves, just applies pressure. A huge resevoir supply is not necessary.

[SSCR]

Just found this post with the search. I have a 1975 CB 360 twin (I know wrong forum) and want to replace my old master. Is my stock 360 a 14mm cyclinder also? If so I sam a Honda master for a dirt bike that is also a 14mm, should this work?

[dpen]

I used a standard m/cyl with my dual conversion & found I had to insert a spacer in front of the piston as the stroke is not quite enough.
  Ended up getting a m/cyl from an early Rollador from a wrecker ($20), whacked a kit through it & the brakes are great.