what exactly does the hydroboost do just make the pedal firmer and apply more braking pressure? I can lock up my tires and the rabs will kick in but it just doesnt stop that fast.
Truck don't stop that fast cause once the RABS kicks in you're relying mostly on your front brakes to slow the truck down. Clamping force in the front calipers is directly proportional to the line pressure you can apply to them, which is where the vacuum booster falls short - a good vacuum booster can crank out up to 1400 psi line pressure, but that's about it, realistically you're looking at 1200 or so. Hydroboost on the other hand is capable of adding about 800-1000 psi over a vacuum booster, so when you jump on the brake pedal you get nearly twice the clamping force at the front brakes, hence the vastly improved stopping performance. There is a downside however, and that's related to not the hydroboost units itself, but the F-Superduty master cylinder - it has a larger bore than the F350 and F250 masters, because the rear brakes of the F-Superduty are discs and their calipers require more fluid volume to be pumped in them than our wheel cylinders do - that extra volume can be achieved with either a longer stroke of the master cylinder piston (makes for a very bad pedal feel), or with a larger bore, Ford obviously chose the latter. Larger bore in the master cylinder makes for less mechanical advantage in the hydraulic system. So if you want to get the best mechanical advantage and most caliper clamping force you need to use your factory master cylinder with the F-Superduty hydroboost unit. In reality you can use the F-superduty master as well just fine, it will make for a bit stiffer and also higher pedal, which actually is quite improved pedal feel over factory setup.
