Parallel bar dips are going to isolate your triceps but one should be weary of the
fact that they are also calling in some chest and shoulders when doing this movement. It is easy to increase in weight when you get to the point that your own bodyweight is just too light for the sets and reps you are doing.
One adds weight by simply strapping on a harness around your waist where you can clip on some extra weight. Be sure that you increase slowly when doing this as it is putting a lot of stress on your rotator cuff and you could injure your shoulders badly.
If you injure your shoulder your whole workout will suffer until you have repaired so it is definitely not worth it. It will affect you whole upper body workout until you have completely recovered as you will not be able to do any shoulder or bench-press.
One can select many different ways to increase when doing parallel bar dips. Anything from pyramids to 4 or 8 sets of 8 to 12 reps will work your triceps to the point of failure. But one should not select a weight that is too heavy as it will cause potential damage to your shoulders as well as your chest.
The parallel bar dips are a great movement that has been done by bodybuilders since the sport started. Doing the movement correctly will isolate the pec-delt tie-ins which give you the added benefit of developing broad pecs which give the impression of size.
This broad looking chest that you are trying to create is what will give you that V shape which is what all bodybuilders are striving for. One should always be weary of the potential damage that you might be doing to your shoulder and do not dip down too low.
The farther you dip down the more you will be working your chest but the problem is that you will also be stressing your rotator cuff excessively so it is better to stop the movement just past parallel. That means that your upper arms are parallel to the floor.
This means that is strongly advised that you take the dip down to the point where your upper arms are just past the point of being parallel to the floor and no farther. Even if you are very flexible and you can dip all the way down without a problem you need to be weary of the potential damage that you can cause.