Question about my air valves and stuff

[jimmydean]

So the dump valve is normally closed. So if I add air to the bag, there is air in the line between the dump valve and the bag, right?

So why is it when I open the drain on my tank do I lose air in all my bags? If I understand how the system should work, the inly time the air should leave the bag is when the dump valve is open.

All I want to do is fix the leak in the left rear valve setup. But I have to put the whole truck on stands because when I drain the tank, I lose preasure in everything. Is that right?