I would operate the machine until the problem appears, and then I would measure the pressures at the brake control valve (located under the operator's seat), AND at the transmission main relief valve (pressure tap should be available outside, at the upper rear of the transmission case).
LATER EDIT - the brake control valve is under the seat, but in order to check pressures, normally the seat shouldn't need to come off. On D6R, you have access to the brake pressure taps from the rear of the machine... if you have a long arm - I think it should be roughly the same on the D6T.
Brakes are spring-applied and pressure-released; the brake control valve and the transmission are supplied with oil from the same section of the power train oil pump.
If there's insufficient pressure only at the brake valve, I would start by taking it off, inspecting and thoroughly cleaning it. If all looks OK and/or the problem still persists, and if you say no error codes are present (meaning electronic control system is OK), then I'm afraid it looks like the final drives (together with the brakes) will need to come off, in order to check for worn brake seals...
If there's insufficient pressure at the brake valve AND at the transmission main relief valve, I'm not 100% sure but I think I would check the main relief valve itself (inside the transmission control valve, under the rectangular cover at the rear of the case) and/or the respective section of the power train pump. I repeat - not 100% sure, but still easier to check than separating both tracks and removing both final drives (including removing the steering diff. motor, which is a pain in the a*$...)...
Also, if the machine has a ripper, it's worth suspending it on the blade & ripper, and checking if both brakes are dragging, or only one of them.
Right now I don't have time to look up / attach procedures & instructions, hope I'll be able to do so later tonight, or tomorrow.
BTW, there seems to be a typo with your s/n - I'm guessing it should be W
FH00364 (rather than W
HF00364...)