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I just got around to running my radiator kit, and it looked like there was air in the system after running it for a bit. So I topped off the radiator through the cap, and put a tiny bit in the reservoir.

After running it a little bit, and giving it some hard pulls to get the rest of the air out, I let it get up to temp. Around 210-220 it began to spit coolant back into the reservoir. So much came out that it overflowed and filled another bottle about the same size as the reservoir. (I was expecting this and had the second bottle setup already). After shutting the bike off and letting it cool down, it sucked all the coolant back in, enough to drain the reservoir and suck air back in. Obviously there was a massive amount of air in the system after the install, but I am fairly sure I got all of it out of there.

I'm not sure if the radiator cap is letting the pressure out too easily or what. The system had no issues before the install, so I doubt the cap suddenly went bad. Has anyone actually watched to see when their radiator cap starts to bleed fluid pressure?
 

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The cap is meant to open at 1.1bar of pressure.

When I pour coolant in a bike, I follow the next procedure:

Always the bike leaning on its side stand (that's why they all got their rad cap on the RH side)
Start pouring coolant in the system. Near the 2nd liter I start squeezing, repeatedly, the bottom rad's hose of the LH rad and the small one on the water pump.
This helps to push most of the air, out of the space close to the pump and in the cylinders water spaces so that when the engine starts the pump can have more powerful flowing current.
Start the engine with the rad cap off and watch the coolant's surface.
When the thermostat opens, the coolant's surface starts rising. If you twist the throttle a bit you'll see that it retreats back down but once the engine is left to idle, it rises again back up.
If you have a lot of air in the system, once the thermostat opens, the coolant's surface goes way down, so you pour a bit of coolant to compensate the gap.
Then you close the rad's cap and rev the bike until the fan kick in.
Then switch it off and let it cool down.
After that, usually, opening the cap you'll see that it needs a bit of coolant (100-200ml) to add.
Then you fill the expansion container to the proper level and you're good to go.
 
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