Wrench cards for a full day and you should come home with ~100 Radiators
But you will run out of cars
Also try the Burnt Forest Biome. Those dilapidated structures that appear all over it have a decent chance of having a brass faucet you can grab with a pick.
But you will run out of faucets.
So yeah, long term Brass is finite. I guess that is why Steel Ammo is in teh game. We have long since switched over the Steel casings because there just isn't enough brass, and since auto turrents only take brass ammo, we reserve all brass we find for those.