The only connection between the brake valve and reverser is that the brake valve is filled with oil from the reverser valve. Applying brakes has no effect on reverser. I am assuming the tractor moved forward before the engine and clutch rebuild? Have you considered possibly the sealing rings(#2a or 2b second diagram)on the forward clutch pack may have gotten damaged when reassembled? If you d remove the reverser valve, be careful of internal oil lines that connect to back side of valve. They can come out of place and splitting between clutch housing and transmission housing is required to get them back in. See items #4, 6, 8, 9, and 10 in first diagram. Of course one of these lines could be cracked or split causing the problem also.