Let me see if I can explain it better. A coolant recovery radiator cap should have two seals. If you look at the under side of the cap, you should see a spring loaded pad with rubber on it. That pad seals to the radiator down below where the overflow tube goes out of the radiator. The spring holds pressure in the coolant system. My original radiator cap for my YM1110D said 0.9 on it along with a bunch of Japanese. That is 0.9 bar or 0.9*ambient - ambient pressure is about 14.5psi, so 0.9*14.5=13psi. You should be using a 13psi cap.
The second seal is inside the top part of the radiator cap and seals on the lip of the radiator above where the overflow tube goes out of the radiator. That second or upper seal is missing on a cap not designed for an overflow bottle.
Here's how its supposed to work:
With engine cold, the spring is short and so the spring loaded pad does not seal below the overflow tube. But the upper seal is sealed on the lip above the overflow tube.
When the engine gets warm, the heat expands the spring to create a seal below the overflow tube. That spring and seal will push against pressure in the cooling system up to the rating on the cap - 13psi. Since the coolant actually expands a little when it gets up to temperature, some of the coolant pushes past the lower seal and goes into the overflow bottle, filling it to the "hot" mark.
When you shut down the engine, the coolant starts to cool, and the spring goes back to its cold length releasing the lower seal. As the coolant contracts with the lower temperature, it wants to draw air into the radiator. Since the upper seal is still sealed, the radiator draws coolant back in from the bottle, lowering the level in the overflow bottle to the "cold" mark.
The 13psi pressure will keep the coolant from boiling as it goes above 212 F. Thats how you can operate the tractor up to 240 F without boilover. If the coolant gets hot enough, the 13psi will not be enough pressure to keep the coolant from boiling, and you get a boil over - blowing steam from the radiator which goes through the overflow bottle. Also, if you shut down the engine while it is very hot, you can get some boiling in the engine block since there is no air flow through the radiator (and due to lack of circulation if a water pump system). That boiling will force steam out through the recovery bottle, and sometimes some liquid coolant with it. If the bottle holds enough coolant, then it will still make up for the lost coolant in the cooling system, but the level in the bottle will be lower.
WOW - that's a long post. Sorry, but I thought I should try to get this one sorted out properly.
