Leaking water from or near a pool filter can have more than one cause. It's important to exactly where the leaks origin is. The leak could be the filter, the union on the pump side, or the union or valve on the return side.
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If the leak seems minor and is little more than a drip, check for any fittings at or above that area that might be loose. Apply a sealant directly to gasket surfaces and retighten.
Streams of Water
A steady or high-pressure stream of water will likely indicate a bad gasket or o-ring. If the leak is coming from either of the pipes that lead into or out of the filter, the union seal or o-ring can be changed relatively easily.
The first place on the filter to look for is the o-ring that seals the top half of the filter to the bottom portion. (Some top-mounted valve sand filters have a removable "cap" instead.) This is a moderately difficult repair.
Repairs can be made to the filter housings, but it is a complex task and a replacement of the failed portion is usually more cost-beneficial.
Be certain to rule out any leak on the backwash or multiport valve or pump as these are separate from the filter. In either case, these are often much less complex repairs. A simple gasket with sealant can solve the problem.