JD400 Reverser - No Forward

Reverser on my JD400 Industrial is malfunctioning. Works in reverse, does not do forward. Checked hydraulic pressures at forward tap and reverse tap on reverser control valve. Pressure is good on reverse pressure tap, there is almost no pressure at all on the forward tap. As clutch is nearly released, see pressure creep to maybe 3-5 psi, then feel a small "bump" then jumps back to zero. I have just changed hydraulic fluid and both filters - all checke done per tech manual SM-2074. Where is most likely place to find problem?