Discussions

So the other day I noticed that my break light came on, checked the fluid and it was a little low. No problem I had some in the trunk and filled it up. A day later it had leaked out enough that the reservoir had gone done half an inch. It is leaking onto the back rims on the inside. My hubby took a look but everything is tight and don't know from where. When I started my car today I noticed it was leaking straight down but without taking the tire off I couldn't see.  
 
So my question to all the smart mechanically inclined people out there is why is this happening? I did change my back breaks but it was over a month ago so why would a problem arise now? My car should still be on warranty so is this a type of thing it would cover? Any suggestions or things to look for would be oh so much appreciated.... money goes too fast when it comes to cars