I believe I did something stupid, help me sort it out.
This weekend I removed the filler cap on the brake fluid
reservoir to clean it. I was on a raid trying to clean up my
engine compartment. I put some cardboard on as a temporary lid.
It's been like this for three days.
Is my brake fluid contaminated, water in it ?
What would I do to "flush" the system and replace the old fluid ?
The lid will never go on again as it was in such terrible shape that
I'm buying a new nice chromed one :)

Thanks / Oskar

I would do a complete system flush to be on the safe side. I use a Mityvac vacuum pump to bleed brakes & it works great.

The good news is that since we're not blessed with anti-skid or other complexities of current brakes the bleeding job is real simple.

Start with the wheel farthest from the master cylinder--right rear, and work your way in to the left front.

If you haven't flushed brake fluid in a few years, it will be pretty easy to tell when you start to get new, clean fluid in the catch bottle. Most bleeders have a clear tube to hook to the bleeder nipple so you can see air bubbles (or lack thereof).