When you hit it with a hammer down it stays down and locks the back wheel
I really hope your just sayin' that to see our reaction...
Take a pic of the rear brake from the rear, so we can see the caliper and the brake pad.
Then have a look at the point where the brake lever attaches to the clevis or where the brake adjuster is.
http://www.youtube.com/watch?v=8Y1wv8Uuezwhttp://www.youtube.com/watch?v=bUAlmm_zf7w&feature=related Remember you have motocross bikes. They can be setup to be ridden for recreation or trail riding.
After you get your current bikes sorted out you can start looking for a good trailbike, that might be a bit quieter.
take a flashlight and have a look in the silencer of your YZ. I'm thinking it probably has never been re-packed.
A good quality packing materiel may help quiet her down a bit, but it's still a race bike. A good spark arrester/silencer
rated for offroad would help quiet things down.
But before we get off track, you should have both brakes working properly. Let's sort out the brake while you're waiting for parts.
Tuck\o/