Well it is OK not to have presure in the hoses when the cap is off. Two stupid questions: The cap itself, is it new and did you try and replacing it? Second: do you have bubbles in the coolant (the bottom hose ont he tank) when you revv it up? As I said, other than clogged system I dont see what elese can blow the hoses up, other than gasses comming in it. It takes a very small hole in the HG, small enough not to cause steam at the exaust and initially you may not see any mayo in your oil. I had the same problem, and it turned out it was a HG after all (prayed that it wasn't but it was). When the hole in the HG is so small it is difficult to trace it. I hope I am wrong, I wish you just have system clogged up. For what is worth you can try and flush or something to see it first. Fluids are tricky, if the flow trough the engine is not right it will overheat in a flash so check everyhting, twice!