The slides release the outer pad pressure, but is controlled by the piston(s). If the piston(s) are not retracting due to dirt in bore, weak hose, (collapsing) or worn boot not holding vacuum, the pressure on the inner pads is greater as you still get some release if slides are even somewhat functional. When replacing Calipers you would be wise to do both sides and replace the hoses as well. Also mentioned was the difficulty pressing the piston into bore....I have found that opening the bleeder on the side you are working on does two things...relieves the pressure and allows dirt out and not back up into the master cylinder.