1999 GMC Suburban I need to refill water every weekend

I don’t know from where my car lost water but every week I need to refill more or less 1 liter.
I check from top and bottom of the car, also radiator is clean.
My Q is, is there possible to lost this amount of water for the cap's radiator?
Also, something very strange happens, every weekend the radiator level is around half, but the small tank where suppose do I need to add water (instead of radiator's cap) is full of water, I check the line between radiator and this tank and is working.
My other Q is, why radiator lost water and doesn't take from Fill's small tank?

Thanks in advance for your help,
Edward