I'm a bit foggy on this....it's been years since I had a Ser III...but I think each filler neck cavity in the body has a drain hole and hose. If the drains are clogged the water could pool up and overcome the filler cap seals.

I know you have new seals, but...... <shrug>....well, that's the only thing I can think of.

Cheers
DD

I would also suspect the filler to tank seal. If the tank is ever so slightly maladjusted it is conceivable that water gets in that way. If the tank neck is not level with the bottom of the filler, or filler neck and tank are a bit off centre in relation to each other it could happen. I would also check the O-rings between the tank and filler neck for damage during fitting. (I assume you fitted new O-rings of the right X-section?    ). I would drain and refill, then just before going out for a drive in the rain cover the filler lids and recess by taping over the whole lot (but use tape that comes off fairly easily and peel it off immediately after the trip).

If it still fills with water I would check the breather arrangement to ensure that is hasn't been connected to the recess drain however unlikely that sounds.