Worn master cylinder seals (pics)

[Bender]

Check the caps seal, it could be letting water or moisture in.

As said 5.1 is the best to use, it contains silicone so it won't absorb moisture as much and work well when heated up from hard brake use. Just flush the whole system with it and chuck it away then refill it all again with 5.1.

[Rippthrough]

And?
Means nothing, the motor factors might have had it years, what's the date on the bottle?

[306Puggy]

Couldn't find a date? Just what looked like a time stamp and a code above it.