In the video you can see you have a leak at the top right where the cylinder is fitted into the block you need a new Cylinder, sorry. Once that cylinder is dislodged from the base a fraction of a mmsi there is no way to fix it. Also Those are the old o ring material, they turn brittle, crack and stop sealing well. The check valve o rings are non proprietary can find at anywhere that carries o rings, a little silicone grease goes a long way.
forms for many reasons but one us the chemical reaction between the water
and stainless and also the nickel bronze
fittings. It's good to remove j tubes and high pressure tube and then fittings and clean them every so often. One speck of rust can get In and score cylinders, pistons, and annular rings.
Be careful when reassembling the cylinders/ bases to the Clark pump body, it is very very easy to dislodge the check valve o rings just a hair and have a leak.