There may or may not be threading under the cap.
Usually, these were welded on the bottom of the cap.
Use a Dremel cut-off wheel or a really narrow saw blade, like a jeweler's "wire saw" to cut through the weld.
Often after you cut it deep enough, you can grip the cap with Vise-Grips and forcibly unscrew it.
The trick is not to cut too deep and cut into the barrel.
Remember: The threads are LEFT HAND and you have to depress the locking plunger on the top side in the front of the sight tower.
If the barrel is threaded, you can clean up the weld and just screw on a replica AK-74 brake. Make sure there's a detent plunger in the front sight, if not, install one.
You'll have to cut a detent notch in the brake.
Another option is to buy an adapter and install an genuine AK-74 type brake which is much more effective then the "replicas".
This adapter fits the standard AK left hand 14x1 threads and converts it to the 24x1 1/2 AK-74 right hand threads.
http://www.k-var.com/shop/product.php?productid=16256&cat=272&page=1
K-Var sells original design AK-74 brakes for both 5.45 and 7.62. Unlike the American replicas, these really work well.
http://www.k-var.com/shop/home.php?cat=344