Komatsu PC88MR-10 Shop Manual page 170

10 Structure and function
Control valve
When boom lowers by cylinder operation
Function
During the boom LOWER operation, if head pressure (B) of cylinder (1) is higher than bottom pressure (A)
and the cylinder is loaded, check valve (2) closes to disconnect the head side and bottom side.
Operation
If the boom is loaded during the boom LOWER operation, head pressure (B) of boom cylinder (1) is higher
than bottom pressure (A).
Check valve (2) is closed by pressure (B) on the head side and spring (3).
The head side and bottom side circuits are disconnected.
10-92 PC88MR-10