Hello All Again... ! another summer in AZ with my Honda Civic 2 dr hatchback has passed without any major hassles... and here came the new one today...

My brake light was ON for a few days and I ignored it thinking it was another case of short circuit as of last time. Thankfully I didnt drive the car much and today I paid off for my ignorance... I went to Just Brakes because my brake paddle just started sinking down to the floor without any significant braking action... i though of brake fluid immidiately but did not know where and why... so I went to Just Brakes nearby to check... what the mechanics there told me was that the master cylinder and a few hoses have to be replaced... (which would cost 551 something... argh ! ) ... so my questions are simple...

1. How do I know if I need to change my master cylinder or is it just the hose...
2. If I have to, is it feasible to change it by myself ?
3. If not feasible, how much money (in reality) am I looking at (to spend ?)

Thanks in advance...
Vin