Upgrade the internal material of the pump for durability. It is constructed from a high-quality alloy that is wear-resistant. The pump is compatible with Graco Sprayer Paint Pump models 390, 395, 490, and 595 (not compatible with ST Max 395/495/595). Please review the instructions carefully before making a purchase. Before installing the pump, be sure to shake it several times to prevent the internal steel balls from sticking to the alloy plate. If you have any questions, feel free to contact us via email for a hassle-free shopping experience.