Best Master Cylinder Size for Front Disc Brake Conversion

[Quicksilver914]

I have a '59 BN6. Just installed front disc brakes. The existing master cylinder is, to say the least, very uninspiring. I can get a 3/4 or 7/8 master.
Does anyone have an experience with one or both?

 

[red57]

Stock for early 3000 front disk brake systems was .625 master (same as clutch). This works well but has a somewhat long stroke.

Stock for the all drum brake cars was .750 where a larger volume of fluid was needed.

For front disc cars, a lot of folks use a .070 master which as a bit shorter stroke but requires a bit more pedal pressure.

The .750 when used with front disks is even shorter stroke and even more pedal pressure (IMO, too much pedal pressure required).

Genuine Girling masters in all 3 sizes are available at Pegasus. Pegasus

 

[Randy Forbes]

Thanks for that Dave; I've never been happy about the pedal travel since I converted the BN6 to front discs from a BN7. I thought I was doing it right when I ordered the same size cylinder that the early 3000/non-servo cars used.

I believe I will go ahead and order the .70 m/cyl, then fit it the next time I feel that life is sailing along too smoothly (it's always such a joy to replace the m/c and bleed the circuits on a Healey...). Honestly, reverting back to the 3/4" is tempting, and I do love a firm pedal (getting used to the power brakes in the M cars took some getting used to).

 

[WHT]

As Dave said, smaller diameter master cylinders require more pedal travel to displace the same volume of fluid as larger diameter cylinders.

Also important, some replacement master cylinders based on the Girling design can have shorter strokes than genuine Girling master cylinders. For example, the Wilwood GS master cylinder and other aftermarket cylinders only have a maximum stroke of 1.250-inches while Girling master cylinders have a maximum stroke of 1.375-inches. Sometimes, the additional 0.125-inch stroke of a Girling master cylinder is desirable.