Push button under the power cord cover on a VAX4000

Hey folks; My recently acquired VAX has started acting up, seems it doesn't always like to power on. When I hit the main switch, the PSU light comes on (orange, by the switch) but the DC OK light does not come on, nor does the chassis power up. I've found that to get the PSU fully powered up I have to turn it off, and push that momentary switch button under the power socket on the PSU. On the VAX there's a sliding cover thing, which when down allows you to plug in the power cord. When you slide it up, it covers the power socket, but allows you access to the thumbscrew which holds the PSU in. When you lift this cover up to gain access to this screw, the cover pushes on some little button under there. What on earth does this button do, and why is it my VAX won't power on until I hit it? :P My thanks all; JP