I've build a few 4-piston setups with stock 19mm master, and am happy with the performance and find it to be civilized. I prefer the smallest MC possible for higher line pressures and less pedal effort. For buses I use the stock single circuit MC with residual valve removed. Here's a bolt on solution for 20.6mm MC:
http://www.cip1.ca/ProductDetails.as...de=C13-16-9554
Cheers,
Lanner