If your brushes have worn to 7mm and you don't feel like dropping everything and getting a new pair right then, you might be able to do this.
Unsolder the screw lug and stretch the spring 8mm longer than it currently is, and re-solder. The 8mm should put pressure at the end similar to what a new brush of 15mm has. Stretching the spring more than that might cause the brush to wear too fast.
I don't know how much of a gap is between the brush holder and the rotor rings. Be careful not to let the brush wear to the point where it can jam in that space and wreak havoc.
Unsolder the screw lug and stretch the spring 8mm longer than it currently is, and re-solder. The 8mm should put pressure at the end similar to what a new brush of 15mm has. Stretching the spring more than that might cause the brush to wear too fast.
I don't know how much of a gap is between the brush holder and the rotor rings. Be careful not to let the brush wear to the point where it can jam in that space and wreak havoc.