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.