The OE boost sleeve in GM Powerglides wears from repeated oscillation of the boost valve. The wear allows leakage, reduces line pressure and contributes to line pressure instability. Additionally, the original steel parts are also prone to rust damage in vintage valve bodies.
Sonnax boost valve kit 28999-01K is the largest factory size (.489" dia. at the small spool), and has a lengthened critical spool diameter to provide better hydraulic sealing capabilities.