When the pump is off, there is no pressure. Kill the pump at the circuit breaker, not just at the timer.
If you want to drain it, open the air bleed. It's hard to tell, but it looks like there's a black wing-nut thing like a radiator petcock behind the gauge. That would be it. If it is, be gentle. And it may screw in to open, so try both ways before you start really wrenching on it.
You'll hear air sucking in and the glug-glug glug as it drains down.
If the lid sticks, resist the urge to ram a screwdriver or some other wedge between the shells. You might deform things and then it will never seal properly. Post about it. It may take a while to get a reply, which is why you want the power off, so the timer doesn't come on while you're waiting.
