If the res was full and now it ain't obviously it (the fluid) went somewhere...
1) where it went is problem one
2) running the res empty has NOW introduced air into your system making for an additional problem of it's own.
How long has the bus been sitting?
I'm guessing you have a vacuum booster/ power brakes?
Remove the line from the booster and see it it's wet inside? It should be dry.
If the booster is leaking it will suck (and burn) the brake fluid into the engine.
https://axleaddict.com/auto-repair/B...-Booster-Check
First top up the system and bleed the brakes. You need to get back to 'working' before you can get to testing...