With all this talk about whether you should use discs without a 
servo/booster or proportioning valve I thought I'd let you know that my '73 
has this set up and it works fine. These are the stock brakes for this model 
year, although a servo was available as an option. BTW its a 1600 which may 
have something to do with it (doesn't go fast enough to need brakes). All 
disc-braked buses sold across the pond may have come with a servo as 
standard to go with their bigger engines, I'm not sure.