The oil level is low.

Add oil in order to acquire the proper oil level. Refer to Transmission Fluid Check .

The external filter is clogged.

Replace the external filter. Refer to External Transmission Oil Filter Replacement .

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

Install a new seal ring onto the top end of the intake tube.

The main pressure regulator valve spring is weak.

Replace the spring. Refer to applicable Allison Transmission Service Manual.

The control valve body is leaking.

Replace or overhaul the control valve body assembly.

Refer to applicable Allison Transmission Service Manual.

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

Replace or rebuild the valve body assembly.

Refer to applicable Allison Transmission Service Manual.

The oil pump is worn or damaged.

Replace or overhaul the oil pump. Refer to applicable Allison Transmission Service Manual.