School buses of that period had the same braking system as a truck of the same model number,single master cylinder,single or dual Hydorvac boosters and two brakes cylinders per wheel.I'm assuming the bus specs would be the same pretty much in any eastern state as here in NY.I believe medium trucks had a single master into the early 70's
I always used a power bleeder,didn't bleed the Hydrovacs separately unless there was a problem after the brakes were bleed as mentioned.Trying to hand bleed brakes on a long bus is very difficult at best.
Of course,the Hydrovacs or master cylinder may have a problem.
