I'm losing coolant. I've replaced the radiator, thermostat, and all of the hoses. I think the gasket might be blown or the water pump has a leak.
A pressure test of the cooling system will reveal if the coolant system has a leak. If the gauge drops during testing, there is a leak - either internal or external. If the leak is external, it can usually be spotted while the system is under pressure. If it’s internal (i.e. a failed headgasket or cracked head) further testing will have to be done in the form of a block test. Using a gas analyzer in the cooling system is another way of testing for internal leaks. A qualified technician from YourMechanic can easily perform these tests and diagnose your problem.