The rundown:
2.1L 89 manual.
Coolant system drinks a cup of coolant every couple of weeks, seemingly
whether I drive it or not.
I did notice a tiny hairline crack in the top of the pressure bottle.
Yesterday the coolant light came on while pulling up a hill, fully loaded,
running A/C, but the engine temp stayed constant.
So today I have removed the pressure cap and here's what I find:
a) gunk in the cap http://www.flickr.com/photos/el_pedro/4735834940/
b) pressure bottle down to half capacity
c) coolant in the bottle has the faintest sheen of oil on its surface - just
that shininess of a puddle after the first rain.

So... is the crack the source of my woes? Is the crap in the cap a sign that
it has failed or is the gunk coming from a serious situation which I
hesitate to name?

Obviously I'll replace the cap and bottle both, but I'm curious to know if I
should be eyeing other issues. The engine was rebuilt maybe 4000 miles ago.

Now to unpack the van and pack less than half of what we need into our
little Reliable Modern Japanese Sedan for "day camping".

Cheers,
Peter