The oil level is low.

Add oil in order to acquire the proper oil level. Refer to Fluid Capacity Specifications .

The external filter is clogged.

Replace the external filter.

The seal ring on the intake tube is leaking or missing.

Install a new seal ring onto the top end of the intake tube. Refer to Allison service manuals.

The main pressure regulator valve spring is weak.

Replace the spring. Refer to Allison service manuals.

The control valve body is leaking.

Replace or rebuild the control valve body assembly. Refer to Allison service manuals.

The valves of the trimmer relays and main pressure regulator are sticking.

Replace or rebuild the valve body assembly. Refer to Allison service manuals.

The input charging pump is worn or damaged.

Replace or rebuild the pump. Refer to Allison service manuals.

The priority valve is missing.

Replace the missing valve. Refer to Allison service manuals.