ASTRO VAN 86' no brakes !?

This is my 'project vehicle' that ran fine at the time that i parked-stored it. After 6-7 years it started and ran fine after replacement of rubber hose in gas tank and battery HOWEVER the brakes went to the floor. I then replaced one brake line after another until they were ALL replaced along with rear drums and and drum cylinders. I then BLED all 4 lines and still the pedal goes to the floor w/ no resistance.> I see NO visible leaks. > this has NO ABS. I see a round black unit from firewall I believe is the Power ist> that attaches to the silver Master cylinder. The key to this puzzle is what might rot or dissolve over time like the hose in the gas tank.....I see in the manual the MC does have 'rubber seals' on the piston.......is that the culprit ? Keep in mind that brake fluid DOES push thru the lines. > but it doesn't have or build any pressure....what are known places in the system that can leak ? will fluid always be seen ? what could a shop offer that i can't do on my own ?