The trap on the pedistal sink should be installed on the horizontal line with a 1/4 inch pitch from the wall to the arm of the trap. There should be a vent inside the wall that leaves vertically from the tee of the trap and heads up to the roof (or ties into the toilet vent). If the sink does not drain well, you should be able to remove the u-bend from the trap and see if there is anything in it. If there is nothing in it, your line from the sink may be clogged. With the trap off, its easy to snake out the drain. Most lav drains run right over and tie into the waste for the toilet. So you do not need a long snake. Even though the sink may have been changed out, a techincian may not have cleaned the drain at the same time. Or, something may have broken loose and clogged up the drian when the tech was working on it.