Things to Check
1. Broken Radiator Cap
2. Use Distilled Water only
3. 50/50 Distilled Water and Coolant or Premixed OEM

As i experienced po..
Kung Sobrang layo ang biyahe or Sobra init outside temp..
Dun lang dapat nagbabawas ang coolant reservoir..
Di normal palaging may bawas.. City driving di pa sakin bumabawas..
Misan lang 2 weeks after.. konti lang from the MAX..
Kung dun sa 3 things to check nagbabawas parin..
pacheck nyo na po sa Aircon Repair shop..
Ingat sa TAP water..