I have a leak that is running down the inside of my firewall on down to the floor pan. I'm pretty sure it's the master cylinder. Is there a seal in the rear that I can replace or should I replace the master cylinder.
IMHO when you think you have a leaky master cylender, replace it! do not try to fix it.
seal's fail for a # of thing's but the most comon is because the part is old and tired.
they sell rebuild kit's for master cylender's but i like to play it safe and get a hi quality new one or rebuilt one.
now is a good time to flush the brake system. this is a thing most people do not do. i recomend when ever you replace the wheel cylendr's or caliper's or master cylender to flush the system with new fluid.
when a break part fail's it normaly contaminate's the fluid. this is a bad thing.
by the way it is probably the rear seal on the master cylender that is going out, i bet your master cylender is going south for the spring. but i am guessing you have described the clasic thing's that happen when it go's bad.