Water & Oil leak

Finding out the parts in my junk box, an used thermostat for Peugeot, I refrained from replacing it for BC7 SUBARU Legacy LSi US model, because of the sealing problem. SUBARU genuine one has a rubber sealing outer round edge portion. I would postpone the work until I receive new SUBARU genuine one.
Before setting it in the place, I would try to ascertain the function by putting it in hot water to activate the wax pellet mechanical actuator preventing the error by an early failure of new parts.

My intimate friend who belongs to OSAKA SUBARU Tezukayama dealer had some problem on his own car after giving it to his son, rear left brake caliper dysfunction. However hard he may pay attention to removing the bubble in brake pipe line and succeed in doing that, the brake pedal comes down relatively soon. And actually some piece of bubbles are recommended. He is repeating the same procedures several times without determining the cause of bubble production.

I just remembered that the bubble exists just very near part of breeder screw, and the oil leak on the breeder causes a bubble production inside. Exactly the same phenomena were experienced when used brake hoses were invited and the used ABS unit was burdened in BC7 Legacy.
The countermeasure for oil leak was inserting thin aluminum sheet as mentioned before, I must tell him about this and I'm prepared for sharing the sealing packing if he wants.