Coolant leak

Logic says that if you lost 1 and 1/2 gallons in 300 miles, changed the cap and then drove 200 miles with no loss of coolant then the cap solved the problem. However, the loss of that amount in 300 miles should have let some traces yet you said that there was no signs or smell. Another possibility is that when the radiator was worked on they under filled it or there was an air block in the system that worked its way out during your 300 mile trip.