Best Girling Master Cylinder size

[Velodrome Racer]

Hey gang-Quick question. I have a 1972 Merlyn Mk20AS, I have enjoyed racing it and learning FF after 15 years with the Bugeye, but I have noticed that some items on the car perhaps, should be looked into before just replaced. My example-I need to replace the front brake master cylinder -and it is a Girling, I have not yet checked the bore size, but other than just matching it, (as it could have been replaced with the incorrect size), I wondered if this group would know what the size should be for the front and rear bores of each of these? It appears I have the Girling LF14 calipers front and rear-number DA21768 at each. As I replace the pads with new-should I replace the rotors too-and I assume those are easily done with Triumph Spit MKIV ones?

[kea]

Master cylinder size. Smaller front then rear.
Probably 5/8" bore front, 3/4" bore rear.
The brake disc may not be a street item.
Often (not always) those street applications used brake disc that had a smaller OD then what we run on our FF.
Besides having Girling masters, I also have KFP Magnum brake pads for your application.

[Dave Harmison]

I highly recommend buy Girling. I have been very disappointed with Wilwood so don’t go cheap.

[Garey Guzman]

Kieth has the size data and Dave makes a great point about quality.
I will add that if I am happy with the balance and performance of my systems, which ever one I'm working on, I exchange like for like to keep it the same. If something is changed, it will likely require a fair bit of focused attention to get back in the sweet spot.

[Jerry Kehoe]

I prefer the AP master cylinders as they have less play before the piston exerts pressure since the port that closes off is in a slightly better position. I like a pedal that is hard completely from the start so I have found this works better for me. The Girling units work fine and are cheaper.