It might help if you posted which cars/mods you are talking about. Some of the default cars do have horrible brakes.
Increasing the brake ducts will not necessarily increase braking power. Brakes need to be in a certain temperature zone to be effective. Too high temps (too small ducts) and braking power drops off. Too low temps (too large ducts) and the brakes never get to work properly. Download the tool XD for rFactor. It will let you monitor brake temperature zones in real time, with a colored HUD display.
Brake balance needs to be correctly configured. If you lock up with the rear, the brake balance needs to be moved to the front. Ideally all four wheels should lock up simultaneously, but since that is a bit hairy to control, you can move it a few clicks to the front, once you have found this setting.
So... with that sorted... you should be able to mash the brakes and get a lock up after a few seconds. Brakes that are correctly set up will allow you to go close to 100% very quickly, but then you need to gradually lift to avoid a lockup. 100% braking power might make this harder for you to do. In that case reduce a bit, until it works for you.
You also need to use engine braking. Downshift in such a fashion, that you have to throw in a bit of throttle to stabilize the car. That is proper technique. Differentials can also help stabilize rear lockups, and can therefore help with braking power.
Good luck.
P.S. if your contact patch (tyre camber, pressure, toe-in/out and caster) is incorrectly configured, none of this will help you.
It's only after we've lost everything, that we are free to do anything.
Edited 2 time(s). Last edit at 12/20/2009 05:13AM by Morbid.