What you have is a hydraboost system for your power brakes getting boost from the power steering pump. On older buses it would have been a vacuum booster working over intake vacuum.
The hydraboost system is pretty much identical to the hydraboost systems used in C/K2500 and larger trucks, vans, and SUV's. The only difference on your bus is your hydraboost system also has an electric override system that will power up the booster if the engine should quit or the power steering pump should fail.
It sounds as if the seals in the power booster have failed. Swapping out a power brake booster is not all that different from swapping out a power brake booster in a pickup. It is just bigger and has a few more connections.
Brake Systems, Inc. in Portland, OR is one of the best places to get replacement boosters or your own booster rebuilt. https://www.brakesystemsinc.com/products.html
A photo of your actual booster would be helpful.