Unfortunately, to me, it sounds like you have a leaking K1 piston seal rings or worn clutch discs. What is happening is more fluid is required on that circuit to take up the extra needed volume. You can test to integrity of the piston seal by removing the valve body and using compressed air to that circuit. Its somewhat of a mute point since whether its a seal or clutches, a disassembly of the transmission is in order.
136 is the K1 drum assembly
147 & 150 are the K1 piston seals
177 on the far right of the picture is the snap ring that's available in different thicknesses that repair facilities will sometimes use to compensate for worn clutches instead of replacing them.
