Zero pressure system. I found this.... The liquid travels into the catch-tank. Once the engine is finished pulling a hill and the system cools down a bit again, it will make a slight vacuum at the reservoir and suck the liquid coolant that has built up back into the system. This is why the catch-tank is fed at its bottom, and also why I had to seal off the coolant cap totally. The coolant cap normally will relieve any bit of vacuum, kind of like a check-valve in the system so it had to be blocked. Otherwise the coolant will not get sucked back into the system and just keep building up in the catch-tank.
could that later be the case?
