With 1 or more tires locking up, it's no wonder you don't feel the braking power you expect. As voltage increases, so does braking torque. But as soon as a tire(s) lock, brake torque drops dramatically. This illustrates the difference between static or stationary friction, and dynamic or rolling friction.
This is why any good proportional brake controller will have you adjust the brakes by (after warming them up):
1. Do a 25mph "snub" apply, with FULL manual apply lever (not the brake pedal).
2. If the brakes do not squeal under FULL manual apply, increase voltage slightly.
3. Repeat 1 and 2 until you get some squeal.
4. Once you have some squeal, decrease voltage until the squeal just barely goes away, under FULL manual apply.
It's important that this is done under full manual apply, because this perfectly set amount is what will be sent back whenever you slam on the brakes in a panic situation. In that moment, if set properly, you will get the maximum amount of brake torque, which is just before wheel lockup.
OH, yeah, like he said, make sure the brakes themselves are properly adjusted first.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller