The OE boost sleeve 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 offers a replacement boost assembly, 28999-01K for the aluminum Powerglide. This high performance design is the largest factory size (.489″ dia. at the small spool), and has a lengthened critical spool diameter to provide better hydraulic sealing capabilities.

• Large boost ratio
• Lengthened critical spools provide better hydraulic seal