That isn't really a bus many here would be familiar with, I would suggest that you talk with local truck/bus shops, it may be a problem with that model. I have had foot valves fail in a short time for no apparent reason.
It may also be something else causing the problem, I did a quick search and found this.
It is written for a semi, but you should be able to weed out the part that pertains to a vehicle not pulling a trailer.
I just found some info, that comes from Australia, It is a pdf download that I don't know how to link to. Do a search for
air brakes, recurring leaking foot valve
. In my search it came up at the top of the list It's from a site