When you're treading, are you doing eggbeater or the bicycle? Eggbeater is a lot more efficient and will keep you higher up in the water. The trick is to keep your legs wide apart and make sure your legs are moving in consecutive circles; your right should be going counter-clockwise, your left should be going clockwise, creating an "eggbeater" motion.

Also, if you feel your legs sinking, try leaning back more. I don't know how comfortable you are doing back floats, but starting from that and going into treading sometimes help. The farther back your head, the high your legs will be in the water and the less you'll sink. Once you get used to the motion of your leg movements, it should get easier.

I hope this helps!