Well I don't know how powerful brake servo is needed but I use a tiny micro servo and it manage to brake my 4000g+ converted Inferno GT1
Only negative is that I only have front wheel brake because I use diff but it work good although.
Kyosho Special Brake Disk (IFW122)
Kyosho Optional SP Brake Pad (IFW330)
Kyosho 16.5mm Brake Disc Bolts (IFW324-01)
Savöx SH-0256
Speed @ 6.0V: 0.16 sec/60°
Torque @ 6.0V: 63 oz-in / 4.6 kg-cm
Weight: 15.8g
Bearing: Single
Type: Micro Plus
Gear: Half Metal
Case: Aluminum
Output Shaft: 21T Spline
Case Size (LxWxH): 22.8x12x29.4mm
