Yes air leaks would be where I would be looking too. So many old hoses, brake booster line is probably the culprit, there are a few joins on it, one way valves etc, at each junction the pipe thins. I used a some 13mm heater hose over the rigid pipe to stop a lot of leaks on mine. Mackay Heater Hose - 13mm, 1.5m | Supercheap Auto