'99 G2500 brake problem

Went to a local market(thankfully rural roads), when I came out of the store and started the van my brake pedal went to the floor and the brake warning light came/stayed on. There were no prior warning signs, just an immediate 80-90% loss of braking that, had I been on a highway, could have been...bad news.

I was able to get it home, looked for noticeable fluid leaks, low brake fluid level, etc., but did not see anything that I could immediately point to.

(unfortunately the temps here have been below -0... so have not had a chance to do a more thorough check)

I know the van is almost 14 years old, but it has less than 60,000 miles on it(few of those over the last 10 years)so been driven very little, especially during the winter(salt)months(not to mention stored in the garage for a few of those).

Reason I bring this up(related or not) is...I've been reading quite a bit lately about GM frame/other rust/corrosion issues leading to brake-line/other problems from late-90 early 2000 vehicles, these are issues which presented themselves to many when the vehicles were only a few years old. I must admit, I've seen old cars that have sat in fields for decades with less rust than what I'm seeing on this vans undercarriage.

Anyway, in regards to the braking issue itself, any idea where to look/test first?
(obviously this thing will remain parked until fixed)

Thanks

1st, is the master low on fluid? If so you have a leak. If not the m/c could be bad, but a thorough diagnosis is required here.

Well things warmed up a bit here so I was able to do a more thorough check of the brake-lines/other. Looks as though I've got a leak on the metal brake line(s) that comes out of the M/C, travel down the "very rusty" frame, turns, then connects into a (control module?). The actual leak (appears?)to be in the area where the 2 lines are fastened to the rusty frame by a plastic/other connector just before the line turns towards the module, and the location is just beyond the driver-side/front wheel-well.

I know it would probably be cheaper to just make my own line, but wondering if there is a pre-fit OEM available?