Thanks for comments!

I did a further check of my system and found the rather disappointing fact, that the HPI mixer will move the reverse servo without notice - not only when the brake has been activated for 3 sec! It seems to happen when I pull the throttle in a lively manner (without braking). crap..

So, I took out the mixer and used a Y cable so the brake and ESC is on ch2 and ch3 is dedicated to the reverse function. It works perfect. Except I have to be carefull not to push the ch3 button when the car is moving!

This leaves me ready to continue, throw in some more lipo cells and get more speed. I am excited to see if if the 3 speed gearbox is any good - otherwise I do have a Kershaw mount lying around that I could use.

I find it really neat to have the servo operated brakes, instead of using battery power. Should give me a little more running time, right?