Bubble by water reserve container

Hello everyone I have a Nissan Xtrail that I just did overhawuld and rectify both cylinder head and block the car starts everything well but I have noticed that a small bubbling is doing for the water reserve vessel and I do not know how to be passing a help or advice thanks ......

It is very important first that the engine is cold and purge the refrigeration system, it is not going to be that the engine is internally air and that is the problem.
When an engine is driving bubbles by the water container, it is many times an indication that is passing the engine compression towards the refrigeration system.
I would recommend first of all that you perform any tests, such as measuring the compression and carrying out acknaked adjustments and if it is at your fingertips to make an empty test to the engine.
If the blowing is minimal you can make some sealant to the radiator to avoid leaks (it is called seal cracks).
First check if the leaks is constant, if so, your cylinder head is torqueda, you will have to disassemble and check if it is crooked

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