Originally Posted by macgyver
Try bleeding your brakes... get some fresh fluid into those lines. Old fluid is bad...
Always start with the wheel furthest from the master cyl and work your way closer. Be careful not to suck air back into the lines again or you have to start all over again... (Been there, done that... It SUCKS)
Words of wisdom to be sure!
While it wasn't a school bus (it was a 1986 Pontiac Parisienne Safari station wagon), it had electro-hydraulic brakes just like you are describing. I experienced a similar problem with it that indeed turned out to be the electromotor. A new one would had to have been special ordered for just over $300.00 (back in 1996) and was not affordable to me.
The solution was to go to a scrap yard and take a power brake booster system off a junked vehicle (of similar model) and replace my electro-hydraulic system. I got the booster, master cylinder, master cylinder plunger shaft & brake pedal (my old brake pedal & shaft were not compatible with the booster system) and the vacuum line that goes from the engine to the booster all for $75.00. It only took about 4 hours to complete including removing the parts from the scrap yard vehicle and my vehicle, rebuilding the master cylinder, and installing the works in my vehicle. Worked like a champ (until the engine blew 2 years later). I also used the one-man brake bleeding kit I got from JC Whitney years ago and performed the sequence Macgyver correctly posted.
If someone is knowledgeable on this system on a bus, perhaps you could recommend a suitable "donor" vehicle he could convert his system to.