Service

On the 21/09/2015 I sent my car in for a 4th service .I have been sending in my car to this branch for the past three years without any problems .Two months after this service I have been getting a message from the car telling me the Brake Fluid level is low and should send the car for a service. I did make an appointment last week and was told they are booking it in today (08/09/2015).I have sent the car in today and I was told I am not under warranty anymore and that I would be paying for this service. My question is, the people who where doing the service were they not suppose to check the break fluid tank during the service in September ?<br> I have entrusted my car to them ,they are supposed to provide a good service to people like me and I feel that they have failed to that .