New mongoose 20'' bmx. trying to adjust the brakes???

First turn the adjuster on the rear brake handle all the way in and make sure the slots on the adjuster and set nut are lined up, pull the brake cable out and leave it attached to the lever to open the rear brake pads, loosen the bolts on the tire and pull the derailer up to remove the tire, remove the dustcap from tube and using a flat tool (Wal-Mart has tire tools for $1) take one side of the tire bead out of the rim, pull the old tube out, place the stem of the new tube in the hole and put the cap on, tuck the new tube in the tire trying to keep it flat as possible, push the bead of the tire back into the rim. before inflating the tire, put it back on the frame while pulling the derailer up to get the chain back on the cogs( they will adjust later) tighten the bolts and inflate the tire to desired pressure (between 35-60 psi)

undo the lock nut that is on the cable holder on the brake. turn the case out to adjust the brake lock up nut when done.

There are multiple possible reasons:
1. Brake caliper is not adjusted so pads are within 1/8" or so of rim (rim should be straight enough to allow that).
2. Brake pads are old and hardened.
3. Too much friction in brake cable and housing
4. Brake cable housing is too long, allowing too much flex.
5. If brake levers have been replaced wrong levers were installed.

p.a. It is helpful to know whether a problem is new, has developed gradually over time, or has always been there, and if any maintenance or event occurred just before the problem began.

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881

Any Mongoose dealer or bicycle shop should be able to handle that or just web search for a 20" BMX wheel. It doesn't appear to be unique.

Try and move the whole wheel back a little and see if that helps. if not adjust the brake pads untiill the contact the rim and not the tire

Yes,as the cable is tighten (brakes applied) the metal slides alongthe cable. hope this helps

The sleeve surrounding the outer cable where it exits from the brake lever mechanism is usually a screw adjuster. Turn anticlockwise to bring blocks closer to rim.

Here is the perfect site for you:http://www.bikeguide.org/how-to/brakes/brakes.php They're called gyro brakes by the way.

you can titen the cable down and that will put more pressure on the tire.

One side of a center-pull brake can be adjusted by tightening or loosening the cable itself (the side with a screw holding the cable in place)
The other side will have a small screw to adjust the tension on the spring.
Sounds like you need to adjust your spring tension screw.

Give it a full (360-degree) rotation and work the brakes a few times. If the problem is worse, then you need to adjust it the other way. If it's getting better, then you're getting closer.
Make several small adjustments, and work the brakes a few times after each one, and you should be all set.