You may also be having the same problem I've struggled with on my kongs. That is, you may be diving too much forward, and not punching up enough.
If you've spotted your hand placement at the end of the obstacle, it's easy (and natural) to dive straight for it. Unfortunately, this results in your hips being too low in relation to your shoulders and your arms to absorb the impact and stop your momentum (acting like brakes, as you've put it).
I'm not suggesting that you should jump straight up and drop directly down on the obstacle; that would be something different altogether! Rather, spot your placement and think of your body movement towards it as an arc, rather than a straight line. Jump higher than what you think is necessary while still traveling forward to place your hands correctly.
This may result in your kong being too high at first, but this will be correctable as you develop your skill and your kinesthetic perception (your "body feel").
In addition, remember to block through your shoulders, shoot your knees through the vault, and keep your hips up.
If my perceptions on any of this are less than correct, please feel free to correct me so that I don't continue teaching myself bad habits!