Strange Braking issue with the Esteem

Hi In the last 2 months, twice I have rear ended other stationary vehicles. Luckily both the times my speed was quite slow and owners of other vehicles so good that I managed to get away without any issues. First time when it happened, I was in conversation with my friend sitting beside me, so I tried to tell myself that it was a concentration lapse issue and nothing to do with brakes. More over there was no damage to either of the cars so issue was forgotten. 3 days ago it again happened. This time, my car suffered some damage. Also, this time I was not talking to anyone. I saw the car in front of me stopped. I applied the brakes lightly. The car was in 2nd gear..so speed was no more than 30. I had enough time to realize that I am not going to stop in time so pressed the brakes hard. The wheels did lock, but it was too late. I hit the santro at around 15-20kmph. This is definitely some issue with brakes. Need your help in understanding it. Some more info 1. Both these times, the car was taken out after a 2 month period standing in garage. 2. Both these times, I had already driven the car for substantial amount of distance with regular braking when this happened. 3. Both these times, after the incident when I tried to pay extra attention to the braking abilities of the car, I did not find anything wrong with it. The car in question is Maruti Esteem VXI 2007 model. I have done only 27k kms so far. The last service was oct 2010 and the brake oil has never been changed nor have I explicitly got the brakes serviced. Appreciate your help. Regards,

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