Bosch Parow

09.2018 i had the water pump replaced by Bosch in Parow, The mechanic told me there was no damage to the head gasket. This cost me R 4800.00. Checking the water on a regular basis on the 21.11.2018, noticed the car was loosing water and then discovered the heater hose was leaking, took the car back to the mechanic and R 1000.00 later was sorted. Checking the water again on a regular basis - now in December 18, found the car is still loosing water and the head gasket had blown, the mechanic is telling me there might have been a air lock in the water system? I want to know surely they should have check the water system again and flushed it out before they gave the car back? I would like to know do i have any grounds to go back to the mechanic and tell them that it is their responsibility to fix the car