Brake fluid

I have an 89 240sx hatchback (auto). I noticed the emergency brake light staying on so i knew that my brake fluid was low. however after topping it up, a week later the fluid was low again. i realised the leak to be between the master cylinder and the power brake mechanism (due to peeling paint). do i need a whole new master cylinder or is there a gasket between there that i should change?
any suggestions?

I had the same problem this summer. I found a new brake booster in the junk yard that couldn't have been on the wreck for more than a few months. I just bought a new master cylinder at the time to go along with it, but I do believe you can just get a rebuild kit for it.
http://www.partsamerica.com/ProductDetail.aspx?mfrcode=RAY&mfrpartnumber=MK2053&parttype=229&ptset=A
or
http://www.partsamerica.com/ProductList.aspx?parttype=230&ptset=A&searchfor=Master+Cylinder

You make the choice.
Question: When you take off after braking very hard, for example stopping at the end of an exit ramp, does your exhaust blow blue smoke?
If it does, then the seal between your master cylinder and the brake booster is letting brake fluid into your brake booster. Since the brakes are powered by a vacuum line, this brake fluid is getting into your intake and burning out the tail pipe.

Long story short, remove the brake booster when you do the work, and empty all of the brake fluid out of it. Don't try to save the fluid for future use. Brake fluid is very good at collecting condensation and you don't want that in your brake system or your engine.

Hope this helps out! That's why we're here!

that wont always happen. the booster usually has a seal of some sort to prevent fluid from getting in. ive replaced leaking MCs that didnt get brake fluid into the booster. and if you do get fluid into the booster, you have to replace the booster because itll eat away at all the stuff inside.

that doesnt happen.. but the fluid is leaking out of where the master cylinder attaches to the power break mechanism. is there a gasket there that i could replace? or do i need a whole new master cylinder?

you need a new master cylinder. the seal thats leaking is inside the MC. the only way you can not replace the whole thing is if you know how to rebuild them.

so i ordered a new MC and im installing it on saturday. How would i be able to tell if my brake booster had brake fluid in it? and also do i have to bleed the brakes before/after replacing the master cylinder; and do i drain the fluid completely, or just out of the reservoir which is obviously getting replaced as well?

you need to bench bleed the master cylinder before you install it. research on how to do this, its not hard. the only way to get around bench bleeding is if you install it and bleed all 4 brakes over and over and over until your absolutely sure you have no air. this is very time consuming and risky unless you know how to tell if you have air in the system by pedal feel. but you should still bleed the brakes even if you bench bleed the MC, just you wont need to do it as much.

as far as fluid in the booster, it usually wont happen, but you should be able to see fluid in the vacuum hose. also your booster wont work hardly as well. when your all done with your MC installation, you should have normal pedal feel. if your booster is contaminated, itll be much harder to stop your car.