I built a set of main landing gear brakes on an old goldberg .25 trainer. It is cheap, easy and quite effective. It operates whenever the down elevator is applied. I used a servo with 2 thin cables from the servo arms that pulls a steel lever (with a piece of fuel line slipped over it) on each gear strut that just rubs against the tires. It doesn't lock up the wheels, just adds lots of friction. I like to tinker, and no one else at the flying club I was in at the time had ever seen brakes before, so it apparently is quite novel to have brakes. It does prove to be handy at times. If you think about it awhile you will come up with a design that works.