You've got air brakes on your train. How much PSI will keep the train from moving but not induce a skid? The simple answer.... no one knows because each train will weigh differently. I can provide you with a baseline though. In hands-on research as well as inquiring with many people, it has been determined that about 30 PSI will do the trick. It's an easy number to remember because car tires have traditionally been set to about 35 PSI. If you set it to 30 PSI and find that your train is not slowing down as much as you'd like, just increase the pressure by 5 PSI and see what happens. Hooking up a large load and you're worried that it wont be enough? That's ok, put it to 50 PSI and back-down a little as needed.
Here is a breakdown of feedback from people about how much PSI they run. Please note that the number cars in the consist was not part of the inquiry and thus these results mush be taken with a grain of salt.
15 = 1 person
25 = 2 people
30 = 5 people
45 = 3 people
60 = 5 people
Based on those numbers I'll make the following conclusion. If you have quite a few cars and/or a decent grade downhill, set it higher. Generally speaking, set it to about 30 PSI.