At present I have a 0.725 master cylindr for my puny 8" drums...

I've got a set of 10" drums off an XR4i and the wheel cylinder is bigger. Obviously these brakes are much bigger in respect of shoe area

If I change the master cylinder to a 0.700 what will the affect be? Will the braking force be increased or will the balance remain similar with the increase in wheel cylinder diameter?

I ask as I've already bought the M/C and don't want to buy another one!

Ta,
Steve

Assuming you have front and rear m/c which is how your post reads, i would just leave the original m/c as is and tweak/adjust it using the bias. If you cant get it within parameters using the bias, then change the m/c.

Reason I want to change the M/C is cause I cant stop them leaking. I've bought some with integral reservoirs to sort it out.

Yes it is a dual M/C with bias bar.. and I've no idea how the nut that was on with a roll pin has come off!