Sweet vintage Trek! Love the tall size, too.

Many great suggestions, and I will double echo the new brake pads suggestion as these old pads are, well, really old and thus very hard and without much ability to stop you and the bike any more. And for riding in the rain, it's even more critical. Salmon Kool Stops (orange/pink/red "salmon" color) are excellent for wet and work well as dry pads. They do, IIRC, make inserts for vintage brakes, though I prefer finding pad/holder units that slide into vintage caliper slots yet have adjustable toe (like modern brake calipers).

Enjoy the refurbishing of your new old Trek. Take your time and you'll get to know it even more. Full 531 is special, and vintage large Treks are a companion out on the road.