The Hayward Filter Control Slide Piston Assembly, has four O-rings, three on the outside of the Assembly and 1 inside the assembly. To access this internal O-ring for maintenance or repair, you will have to partially disassembly the Assembly. This guide shows you how to access the Internal Shaft O-ring.
PULL OUT DRIVE PIN - The Drive Pin is in tight so you will have to punch it out most of the way with an awl or small punch. Put a board under the shaft to support the impact so you don't bend the shaft. After you have punched the Pin most of the way out, grab the other end with a pair of piylers and pull it out the rest of the way.
LUBRICATE 0-RING - Inspect the O-ring for wear or damage. Replace if necessary. Then lubricate the O-ring with a silicon or Teflon based lubricant as identified above.