I could not find any description of the problem or a solution, so I thought
I would post my experience here. I am not cool enough to create an
Instructable. It is on topic because we bring raw food home in a cooler,
and take cooked food to friends and picnics in coolers.

The problem is that the lid will suddenly swing open, especially when
carrying the top-button Playmate cooler. The root cause is a spring
that has lost its zing.

Carrying produce back from the farmers' market when the lid suddenly
swings open and dumps its contents on the ground is intolerable.

The lid is held closed by an L-shaped lever that sticks a rectangular
tab into a vertical groove on one end of the cooler -- the end adjacent
to the push-button. The lever is kept in place by a compression spring
between this lever and the lid. The spring fits over a pin on the
closure lever and fits into a pocket mounted on the lid itself.

When the push-button is depressed, it pushes against the bottom of
the L-lever, compressing the spring, which makes the rectangular tab
recede from the groove, allowing the lid to rotate open.

When the push-button is released, when the lid is rotated back to
vertical the spring presses against the L which forces the tab back
into the groove.

When the spring has lost its zing, the weight of food in the
cooler will let the lid open.

To replace the spring, you must pry off the caps on either side of
the lid, unscrew the screws, and pull off the lid. Then you must
pry up and rip off the part of the lid foam that covers the mechanism.

From inside, squeeze the sides of the push-button nearest the end of the
lid, push in, and pull out the push button. Then remove the lever and
take the spring to a hardware store. You want the same diameter spring
(so that it will fit over the pin) with a little more oomph. I was
able to find a spring slightly longer, with a slightly smaller inside
diameter, that I was still able to twist on the pin with the help of
pliers.

Then I replaced the L-lever,with the spring between lever pin and
lid pocket. While holding this in place, I reseated the push-button,
first at the lid end and then at the far end.

Screwing the lid back in place and popping the endcaps back on, I
tried it out. It appears to be secure, but I will have to verify
in actual use.

If this does the trick, I will glue the piece of lid foam I broke off
with Great Stuff polyurethane foam.