lumberjacks right any pressure problems check the small filter in the control vavles on top of transmisson.
Remove top of the transmisson and remove big allan key plug about 19mm. think its in the centre of the block pull out the vavle which has a small filter inside about 2mm-3mm and just see if its loose if so tighten and rebuild.
If it is loose will give poor drive both directions and had it once when only in forward.

i sure someone will explain this better but will point you in the right direction.

i have also had clutch plates burnt out on a doosan which was less than a year old.