ISUZU WORLD KEMPTON POOR SERVICE

My brand new isuzu bakkie use brakefliud in 6 months had to fill the system 3 times as the fliud level drops to empty suddenly. One day it is full the next day empty then stay full for a few weeks then the same thing happen. I reported this to the service department and they said they canf find any thing. I aksed for the workshop manaeger to call me up to now after 4 days no call or feed back