Coolant system does run under pressure in normal circumstances, depends how much tho! Make sure it's properly bled or you'll have an air lock which will cause pressure. Also, check the thermostat's ok.

Re the breathing heavy issue - you're always going to get crank case pressure but again it depends on how much. Does it run ok? If you've tested for compression and leaks and all appears to be ok then I'd get it down to a tuner for an expert opinion