Spring, Pressure Relief Valve, Yak 52
This spring, item 3 in the diagram, sets the Air System pressure for the Yak 52 Pneumatic circuit.
It's rare, but they do break.
Use this Spring for Yak 52 and 18-T - ( their PRV piston is a larger diameter than for single seat airplanes like Yak 50 and 55.)
PRV Springs for those single place aircraft are both lighter weight and have different dimensions.
The correct piston for Yak 52 PRV is available here