Dave, that's right, discs and drums don't go any bigger with hydroboost, however you gotta remember that braking torque is directly proportional to the clamping force of the calipers, which in its turn is directly proportional to the line pressure applied to them - so with the hydroboost you get higher braking torque per inch of pedal travel, and also higher maxiumm braking torque when you're standing on the pedal with all you got. It also makes the pedal feel better to most folks, as it's now stiffer and more responsive than the mushy bowl of jello pedal feel normally experienced with the factory vacuum booster - it's often described as the responsiveness of manual brakes with the assist of power brakes. To me personally this is a great selling point, as in a previous life I used to do brakes development for an OEM supplier and I'm very particular about brake pedal feels and what not. And yes, of course you can burn up the brakes with the vacuum booster while towing, the advantage of the hydroboost tho is that when they start fading due to heat you can apply still a bit more line pressure and somewhat counter the fade for at least a short period of time.
and the F-450 hydroboost unit DOES have a way to provide pressure even with the engine shut off...there's a nitrogen cylinder or something on those lines that stores built-up pressure and can allow the pedal to travel either two or three times with full assist before you lose the assist. At least, that's as I understand it...Calvin or Gary can probably explain it better than I could. I do know that there IS a mechanism included in the unit.
