The motorcycle handbrake is one of the several scrap items that can be used in the construction of weapons using schematics. This item is used in the construction of the Shishkebab, but serves no other known purpose, except as Rock-It Launcher ammunition. They are often found next to motorcycle gas tank. Incidentally, whenever one finds the two together, one should always pick up the Handbrake first - picking up the gas tank will often cause the Handbrake to clip right through the floor.
2 outside houses in Minefield. Each is located near the burned out remains of a motorcycle, along with a motorcycle gas tank. One is on the east side of the Gillian House, and the other is on the southwest side of the Benson House.
2 at L.O.B. Enterprises. Upon entering the Archives sector, go down the stairs and turn left. Keep walking on the corridor until the end(some sort of maintenance area with two generators.) The two motorcycle handbrakes are there, along with two motorcycle gas tanks.