42RE/42RH/46RE/46RH/47RE/47RH/48RE, And A727, A904 Transmission Sonnax Boost Valve Spring Retainer When listed Chrysler units feature a TCC boost valve lineup, the OE retainer is prone to cracking and breaking, allowing the the boost valve and spring to fall out of position. This causes a leak of 4th Gear and TCC oil pressure. Sonnax boost valve spring retainer 22990-01 eliminates this leakage by securely keeping the valve and spring in place. It is made from steel for greater durability than the OE retainer.