Steve is right - could be a simple cause - water in the fluid. The water boils off when the brakes get really hot, and then the resulting vapour compresses, giving a soft pedal. Brake fluid is hygroscopic - absorbs water - and many people say that brake fluid should be changed every two years for that reason.
However, a brake pedal that's firm when you stamp on it and goes softly down to the floor otherwise is most likely the master cylinder.
I vote with the European view (replace not repair) on this one - master cylinders are not expensive over here either really (about $50). There WILL be corrosion pitting... in the unlikely event that the master cylinder bore shows NO corrosion, then I'd get new seals and rebuild. Once upon a time, people used to hone the cylinders and fit different seals. It's just not worth the effort now.
The driving conditions on those beautiful roads in the South are different to anything I've seen in England - it's no wonder that you've found 5th gear not especially useful - I've driven several standard Uno 70s and they won't ever pull 5th gear up those sorts of hills. Meanwhile I doubt that the Chas (1986Uno45S) 70SX red rocket ever has to cope with anything like a 17% 2km decline.
I remember wearing out a set of brake pads driving from Nelson to Takaka. Well, the brake pads were 'marginal' but would have easily lasted 6 months-1 year around town, I burned 'em down in one trip in the South Island
Of course, being in a low gear for descending hills is the best idea, but even so there comes a time when you need the brakes.
-Alex
