Tufty, you're right except that you've said they were "required" to be refilled. More accurately, they were "allowed" to be refilled. Any fluids used during the race could be topped up at the end before weighing. Thi smeant that teams (in particular Williams and Lotus) had a 20 litre (20kg) reservoir for water used in "brake cooling".
As another poster said, these 20 litres were used up at the start of the race which allowed the cars to run 20kg lighter than those who didn't employ such a system.
I had an idea to run sidepods filled with zinc/HCL so as to produce hydrogen. The idea would be to drop the zinc so as to make the car lighter and then at the end of the race, vent the hydrogen to make it heavier again.
Similar (safer) idea would be to fill the sidepods with helium at the start and to remove some ballast during the race and again, vent off the helium at the end.
Unfortunately, I think the amount of helium needed to make any significant difference to the overall weight of the car would have to be in a huge balloon over the top of the car - or at least that's what Patrick Head told me!
I had this argument with one of my apprentices about 10 years ago concerning a rugby ball filled with helium, so set him the task testing it. It turns out that when the ball was inflated to the same 'hardness' ( ignore pressure ) the weight was identical to less than 2 gram, which could be accounted for by air pressure in the lab or moisture content of the gas/air.
So if it is the same 'space' no benefit.