would you mind describing in more detail the nature of the malfuntion? Are the cyls or mast rails mis-staging at any point in their travel, or does everything work normaly until they are lowered from a certain hieght and then just lock up? Also, what type of mast is this? Toyota or some other brand like lift tech? 2 stage, TT, or Quad configuration?

I am not familiar with this toyota model, but I have seen similar problems in other machines. On Nissans with TT type masts I have seen the check valve inside the center cyl malfuntion and cause the center cyl's ram to lock up. Usualy what happens is the ram will only move through the first half of its travel & stop, then the outer cyls start lifting, but I have seen it happen the other way around too, the center cyl raises normaly, but then locks up while it is on the way back down.

I have also seen masts with rollers that were damaged, or shimmed to tightly lock up when lowering with low or no load. Including one location where three machines had been damaged the same way(damaged carriage rollers) by the same operator.