brake pedal hardto press at first and slow to return
 

Recent my brakes have been acting up. When I go to press the brakes it is almost like something is catching and then it goes smooth. Then it would realese extremely slow and if I tried to hold the brakes you could feel the pedal pop so to speak. I replaced the front calipers and it seems to not happen as bad. Now I can drive a while in stop and go traffic and it's ok for a while and then randomly the pedal is hard to push, as if it's hung but will be normal right after I step on it.the pedal does realeas quicker than before but every now and then it doesn't. Before I go and replacing more parts I wanted to get some input on what might be causing this. Any help is appreciated.