Should I change V-brake pads if they are old and look a bit dry?
If yes, how I can determine if pads are dry enough, or not?
Just to be clear, the pads are not worn.
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A rough rule of thumb is that if you press a fingernail hard into the raw edge of the pad, it should deform the pad and go in some.
If your brake pad is hardened by age, ozone, or UV exposure then braking performance suffers.
Your rims will also wear down faster because the pad is harder than it should be.
If the pad is black on the black bits (or a good healthy looking colour if its red or somethign else) then that suggests its okay. If a black pad is grey, or worse faded to light grey or white, then replace them.
For a bike I care about, I'd just replace the pads with known-good brands like Kool Stop and ride it. For a bike that is only a parts donor, I'd throw the pads away and only store the bare brake calipers/arms.
If you choose to keep using old pads, then at the bare minimum pick all the detritus and debris out of the pad - you will be astonished at the amount of metal particles that the average brake pad can hold, and these metal flakes are what contribute a lot to rim wear.
I had an old 80s road bike for a short time, it had single pivot brakes and totally unworn but hardened pads. It braked "okay" on the flat, but a hard brake when on a downhill led to some terrifying oscillations as the brakes grabbed and let go, as the fork flexed. I ended up walking down that hill it was so bad.
Change your pads - they're important safety items.