When I was changing my rear breaks, it was almost impossible to turn the piston? (I believe I'm not to good with cars I just changed my spark plugs on my own for the first time today) i could barley turn it back in. I also noticed when going 50-65 mph when I break, once I get to around 10-20 the car shakes or trembles a little bit,I feel like I may have also noticed a slight VERY SLIGHT pull at this time towards the right or left. I'm going to take a guess I need to change the caliper ? I want to do this myself and I want to use the correct parts (oem?)and have everything I need as in tools, parts etc that I need. Please help