Bhj, long gone have the days of HGV Vacum brakes, these were a system of air / fluid assisted brakes, at that times HGVs did not pull the weights theses trucks have today that includes 8 Wheel tippers, no matter where a HGV is, if that truck has a Air Loss by a leak, that truck will come to a stop and the driver can do nothing about it, that's safety in the making, a lot of people think a HGV will stop dead, it will not, if it's empty it will skid due to the efficiency of the AIr Brakes, Loaded well that's another thing depending what sort of HGV you are driving, a Tanker can "Push you" although there are baffles within the tank they can still push ( the fluid splashes about) they have forward baffles to stop the fluid going forward & Side baffles for corners etc, to cars as you mention, older cars, as you said had drum brake & if there was a leak within the four drums you would run out of brakes, now they have duel, if there's a leak on the back it would not affect the braking & Vice versa, the rear brakes come on secondary, regards hill decent's, you have to be on the ball, before any steep decent you have a warning, Ignore these you pay the price, you mention Sawley so you must know the area, prior to Sawley there is no hills that could affect youtr braking, but talk about Waddington Fell that's another danger area as a few have found out, there's been a safety slip road there for HGV's to run into should they have brake fade, the village of Waddington has had a few "Run Aways" in my time in this area all due to continuous braking. I always drove with one thing in mind "What If"