I showed it to my builder, he says, they won't do that since we
already signed for second walk-in, but before second walk-in, we never
had more rain at all. Now he says, they won't do that, I have do part
of my maintenence.

It not one year we got this new house, how can I approach this, till
one year, builder's responsibility for any leak/damage right?. How can
I fight with him legally since it is not even a year yet?. Any
suggestion appreciated.

Start by posting the biggest freaking sign in your yard that zoning will
allow, stating:
"This house was built by xxxxx,
the basement leaks, and they
won't fix it."