Brake light coming on?

[fender22]

Sorry for the dumb question but noticed a brake light
coming on last week. I'd just been through a creek
so I thought the electricals may have gotten wet. I
quickly checked the fluid level and seemed ok so
hi tailed it home. Noticed again this morning the
light is coming on again, mainly when on an incline or
when I stop quickly. It's a 93 200TDI so imagine it
doesn't have sensors on the disk etc and the light is
low fluid? Obviously I need to check the fluid again.
Do I need to do anything special ie pump the brakes while
I top it up etc?

[fender22]

I just checked the level and it's about an inch off the top of
the reservoir but well over the minimum mark. I topped it up
a bit anyway. Manual says the brake light is operated by the little
contact in the lid of the reservoir so if it's not the level switch
must be dicky