and also you must bleed the whole system each and every time you open it. There's no such a thing as "I only replaced the front cylinders so I only need to bleed the front".
Don't worry over having to bleed all 4 wheels and using a quart or two of brake fluid. Many people don't know you are well-advised to refresh the brake fluid (bleed) every few years. That is to combat moisture absorption. Besides rusting everything internal, the moisture greatly decreases the boiling point, making it more likely your fluid will boil and lose all braking ability. A smart alternative is to use silicone fluid (DOT 5), which doesn't absorb water.
You might get away with 10 years if you live in the dry West. I was trying to bleed my Aries once which had spent 10 yrs in Georgia and nothing was coming out of one caliper. I had the assistant jam the pedal with both feet and a big slug of rust blew out the hole. I have since used silicone in all cars that don't have ABS. Since it is getting rare, I bought a gallon on ebay (military surplus). The only drawbacks are that it has a slightly lower boiling point than fresh DOT 3 & 4 (though they soon degrade) and it is slightly more compressible, which I think is why they say not to use in ABS systems.
