Assess

Run a full water test before you start so you know your baseline — free chlorine, pH, total alkalinity, calcium hardness, and CYA. Water balance calculator
Note how far the filter pressure has crept above its clean baseline — a hard-run pool usually loads it well past the spring number.

Filter deep clean

Turn the pump off at the breaker before you open anything on the equipment pad. Never open a pressurized filter — release the air relief and drop the pressure to zero first.
Deep-clean the filter, not just a rinse: soak a cartridge or DE grids in filter cleaner overnight, or backwash and deep-clean sand — a midsummer filter is full of oils a quick rinse will not shift.
Inspect and lube the pump-lid and filter O-rings while it is open, and replace any that are flat, cracked, or weeping.

Scrub the waterline tile band to remove the summer oil-and-scale ring before it hardens.
Brush the walls, steps, and dead spots, then vacuum the floor to waste if fine debris keeps passing back through the filter.
Clear the skimmer weirs and baskets and check the return-jet eyeballs so the whole pool circulates evenly again.

If CYA has climbed high from stabilized chlorine, partially drain and refill — dilution is the only way to bring CYA down. High CYA quietly weakens every chlorine dose, so a summer refill often fixes a pool that "will not hold chlorine". CYA calculator
Re-test and rebalance total alkalinity first, then pH, then bring chlorine back to target for your CYA. Chlorine calculator