master cylinder for disc brakes

I am using a Datsun 120Y M/C with 11/16th bore mounted direct to firewall with Volvo 4 pots up front & Skyline R31 discs on rear.
 
Matt

Whooligan,
I had to get my conversion engineer certified for rego in Qld as the car had been off the road for some time. He was happy with the setup but suggested removing the valve from the distribution block in the engine bay. The standard setup is 60/40 with the most bias going to the front discs. By removing the valve he said the bias would be 50/50 with the rear discs getting the same as the front. My car was already painted and finished and I didn't want to get brake fluid over everything by changing it at that point.  I still have quite a reasonable pedal, getting vacuum from the twin weber setup was my main problem. I've had to tap 2 inlets to get vacuum which has caused some dramas with tuning. Any suggestions for good vacuum without disturbing the webers?
Cheers,
 
Len

Hi Mud,
Had thought of using a vacuum tank-what sort of dimensions are required for this- I've got a pinto on twin 45's as well. Are they efficient?  Len