Pneumatics have 2 main areas that can create leaks.
First, there are several threaded areas of the system that be loose or leaky.
Second, tubing connections can leak.
Take a look at this diagram of the legacy pneumatics system with a single action cyclinder such as yours.
Go through your system and check every single spot carefully and thoroughly. It’s there somewhere, and you can find it.
Never tape on the tubing! To keep tube from leaking, you need a clean straight cut before you push it into the fittings. On the smallest fittings, into the solenoids and the air cylinders, there are small sealing washers. On the fittings to the air tank, you can use teflon tape if needed.
Mix a bit of dish soap into water to make a leak-test solution and it will blow soap bubbles where the leaks are.