the best thing you can do is trade the Toyota's out and put in the Doosan lifts with the OCD (oil cooled disc brakes) then you will not have any brake issues.
i have had some applications that were very rough and had the same brake issues you are talking about and i had to go to the Doosan OCD to solve the issues.
yes there are pro & cons to any lift truck but in your case the OCD brakes will far out weigh any of the Doosan cons.
This is one of the issues us BT / Toyota engineer's bring up from time to time, customers, salesmen & engineers all keep asking Toyota to drop the autobox / drum brake set up & go hydrostatic, will Toyota listen - NO they wont.
I know several customers have refused to purchase Toyota counterbalances because there not hydrostatic drive.
If I was buying a counterbalance truck with my own money, it would have to be a Linde.
Thought:
Very specific problem... If having blown in a dust really helps it is possible to try to organize tap of a dust from a working surface of brake shoes. To make a type notch \\\\\\on a working surface of a shoes depth no more 1,5мм. A residual thickness of an overlay not less than 2/3. Absolutely a problem won't solve :-( but it will be possible less often.
Won't be worse.