How do you know it needs replacing?
When does it usually get replaced?

cant seem to find an answer.

Cheers

 

That is hapening that makes you think its the master cylinder

 

Usually you'll brake will sink down, whille you brake... and you'll have to pump your breaks to stop...
see that your master cylinder is wet or if there is visible leak on the firewall.....if you have abs your abs light I think will go on, cause its catching air.