US Patent 7467642 Soft ventable relief valve

A pilot operated pressure valve which limits the rate of pressure rise and substantially eliminates excessive system and tank pressure spikes. Because the pilot ball downstream of the damping orifice is held off the pilot seat in the normal starting condition by a pilot return spring, the main piston will open immediately when system pressure overcomes the low bias pressure of the main spring. A pilot pressure bypass port, which selectively deactivates the pilot chamber features, may also be included.