I just put new wheels on my old bike and the braking is VERY bad. It will stop, eventually, but emergency stops are out of the question. Is there a "break-in" period for the new wheels?

FAQ and without any details .. what brakes specifically? Type Brand and Model ..

Try cleaning the braking area of the rims. Also rim/brake pad combinations work better than others

Mike

Did you clean the brake tracks on the wheel rims before putting the wheels on your bike? New rims often have a protective coat on the brake tracks and you need to remove it for the best brake action.

Have you adjusted the brake pads to press flat on the brake tracks? Different wheels often have the brake brakes in slightly different positions at a different angle. You need to make sure that the brake pads press flat on the brake tracks of the new wheels, AND a fresh surface is exposed on the brake pads. You can use a file to flatten the brake pads and remove a small amount of material to expose fresh rubber on the brake pads.

Did you use steel wheels or aluminum?
Steel has poor braking.

More info would be a big help in answering your question.

For example, did you have good, fair or poor baking on the old wheels? What else changed? What are the particulars on the new wheels, ie. anodized rim, machined brake track, etc. Did you also replace or deglaze your brake shoes?

The more specific info you provide, the better the answers you'll get.

If you've heard this question too many times before it might be a sign that you need to move on.