The document provides instructions for disassembling and assembling the piston pump (hydraulic fan) on Caterpillar 336D L and 336D LN excavators. The disassembly process involves removing 30 parts in 9 steps, taking care with springs that can propel parts, while the assembly follows 30 steps to reinstall all parts in the reverse order. Tools include retaining ring pliers, a dial indicator, and a bearing puller. Cleanliness is important and worn parts should be replaced.