1) First thing I'd look for is exhaust leaks.
2) Loosen the bleeder on the rear caliper, position something to catch any fluid, then press on the brake pedal and see if fluid comes out. If not, you either have a clog in the line (less likely), or the master cylinder needs to be rebuilt or replaced (more likely).
If fluid does come out, then the caliper needs to be cleaned/rebuilt/replaced. First try cleaning. Remove the pads and the caliper (you won't have to disconnect the hose) and clean any crud that has built up around the pistons and seals, reassemble and test the brake. If this hasn't cured it, you'll need to rebuild it (if a kit is available) or replace it.
3) If it only does this when cold, I wouldn't worry too much - as long as you're not loosing coolant. There could be moisute in there that can't find the path out thru the drain, either by design, or due to age and the path is now clogged by the dent or with rust/crud.
Be sure you're not loosing coolant, if you are, it's a sign that coolant is finding its way into the combustion chamber, either from a bad head gasket or a crack in the head or block. If it's small enough, in the right place, it may sort of seal itself as the engine warms up, but in these kinds of cases, its just a matter of time until you have a bigger, and possibly catastrphic, failure.