It depends on the solenoid model but usually it's the small spring around the magnetic core of the solenoid. The spring pushes the core against a small air orifice, so if the pressure is too high then the spring can't seal the orifice.
If you put a stiffer spring around the core, the bottleneck usually shifts over to the spool valve. The small seals located around it can get blown out of their grooves when the valve moves, due to the air that's trapped under the seal.
Like I said it depends on the valve make and model, some offer very different designs (like Humphrey) but not everybody..
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As told by Stobaeus
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Used parts f/s