I am so uncertain about this, please forgive if I got it wrong.
I believe the white piston goes in and out and gets scratched. There's a specific lube for the outside of the white piston. A guy posted pictures of his white piston which was scratched.
When I inspected mine there was an intact black rubber boot around the white piston and it felt like it was filled with a lube. I saw no need to destroy a good, non-leaking boot in order to inspect the white plastic piston.
As I recall deep scratches mean the end of the boosters service life.
I realize your problem is inside the white piston. Perhaps application of suitable teflon lube will reveal the fault by altering the leak's rate.
2001 is old enough. How many miles? Remember, lotsa people will be offline until the holiday is over.
Search here and on Matthews for "brake piston", or teflon "brake lube", or "booster piston"
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Our 3rd V70
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