I'm concerned that you may just be treating another symptom, rather than the underlying problem. A significant amount of fluid should flow through that hose with every heating cooling cycle. As the engine heats, the water expands. In most engines the fluid is contained in the engine and is pressurized until it gets to about 14 psi, at which point a valve in the pressure cap (radiator cap in a car) releases and allows the still expanding fluid to escape to the overflow tank. This is why you should never open the cap after the engine has heated. Check the level in the tank when the engine is warm, it should be much higher than it was when the engine was cold.
When the engine cools the fluid contracts, and another valve in the pressure cap opens, and lets the engine suck the water back from the reservoir.
If the engine has a leak in the fresh water system, the pressure in the system will push the coolant through the leak instead of through the pressure cap, and when the engine cools will suck air in through either the leak or the pressure cap or both.
The volume of expanded water in each heating/cooling cycle should easily overcome any hills and valleys in a small diameter hose to the coolant reservoir.
When the engine cools the fluid contracts, and another valve in the pressure cap opens, and lets the engine suck the water back from the reservoir.
If the engine has a leak in the fresh water system, the pressure in the system will push the coolant through the leak instead of through the pressure cap, and when the engine cools will suck air in through either the leak or the pressure cap or both.
The volume of expanded water in each heating/cooling cycle should easily overcome any hills and valleys in a small diameter hose to the coolant reservoir.
