WE'LL LET THE GUYS FROM SONNAX EXPLAIN:
Vehicles with a 4L60-E transmission frequently have poor line rise, which can result in 3-4 clutch, 2-4 band failure or poor shift quality. The duty cycle of the EPC solenoid causes the boost sleeve to wear quickly. Oil entering the torque signal orifice, leaks past the boost valve and exhausts at the reverse orifice. Oil enters the torque signal orifice, leaks past the boost valve, and exhausts through the reverse orifice, resulting in poor line rise. Reverse oil can exhaust both to the sump and back through the torque signal orifice.
The boost valves have been specially hard coated to eliminate excessive wear from occurring on the inside diameter of the sleeves. These boost sleeves have been manufactured from high quality aluminum, specifically designed to resist wear.