A tool box that gets water in it is one of the most common complaints about bed storage, and the gasket usually gets the blame. It is often innocent. The water is arriving another way.
Where Is the Water Coming From?
Check three things before touching the seal: the drain holes, the latch adjustment and the lid alignment. Most boxes are designed to shed water past the lid and drain it out, not to be watertight, so a blocked drain turns normal running water into a puddle in the bottom.
Condensation Looks Like a Leak
A steel box sitting in the sun all afternoon and cooling overnight will collect moisture inside with no leak at all. If the water appears as an even film on the underside of the lid and on cold tools rather than pooling under one corner, condensation is the answer.
The fix is airflow and absorption rather than sealing. A tray with a gap under it, a desiccant pack changed occasionally and not storing wet gear in a closed box will do more than a new gasket.
Test It Properly
Dry the box out, wipe the interior and put a few sheets of newspaper or plain paper in the bottom and along the sides. Then run a hose over the closed lid for a couple of minutes, working from the front to the back.
- Wet paper at one corner points to lid twist or a latch out of adjustment.
- Wet along the whole seal line means the gasket is compressed or perished.
- Water in the bottom with dry sides is a blocked drain, not a leak.
- Wet under the mounting flange is water tracking along the bed rail.
Lid Twist Is the Usual Culprit
Tool box bodies are not rigid, and clamping one down unevenly twists the body just enough that the lid no longer meets the seal along its whole length. It shows up as a leak at one corner and nowhere else.
Loosen the mounting clamps, let the box settle onto the rails, then retighten diagonally in stages while checking the lid closes evenly. That corrects a surprising number of boxes that were assumed to need a gasket.
Then Look at the Gasket
If the seal is genuinely at fault it will look it. A gasket that has taken a permanent set, gone hard or split at a corner is not going to recover, and replacement strip is inexpensive next to a box full of rusty tools.
Clean the channel back to bare metal, fit the new seal in one continuous run and join it at a corner rather than mid-span. Shopping for a replacement box instead? See truck tool boxes, aluminum boxes or crossover boxes.