How to classify accelerometer data?
You should consider doing an STFT on the accelerometer signal. This will allow you to visualize the frequency of the signal. If you can determine the frequency of the brakes, you should be able to set a threshold for that specific braking frequency and count each time it is exceeded. You would also be able to measure how long the braking occurs.

How are you processing your signal? Matlab and python have the specgram function, which will allow you to see the actual frequency braking. From there you will be able to determine the brake frequency as well as a threshold.