How can I fix brake pads being too tight even after pushing the pistons back in?
My avid brakes had the same problem and it was due to too much fluid in the system, to solve the problem :


Remove the pads and keep them away from any oil (fluid). 
Unscrew the bleed screw a little and put a towel on it
Push the calipers back completely 


4.Clean calipers well before reinstalling pads. 

Be aware that the amount of fluid that will exit should be a drop or a little bit more. 
Let too much fluid exit and you will have to rebleed them, so do it gradually.

This is a common problem on new avid sram brakes.