need help with malfunctioning brakes!!

[gold84gmc]

hey, the truck is an 84 gmc 2500 2wd with the HD brakes. a few days ago i went to put the brakes on and they went almost to the floor and it acted like only the front brakes were working; i checked the fluid level and the back resevoir was about half empty so i topped it off. and then they seemed to be working alright. and then yesterday they started doing it again, and then on my way home i went to push on them and the pedal went to floor/maximum travel...so i had  to use my parking brake to stop (and it was also going down a very steep hill). when i got home i checked the fluid and it was still full. i checked the entire system and couldnt find any leaks. what do you guys think it is? im planning on replacing the booster, and then the master cylinder if the booster dont fix it. so your help would be greatly apprectiated.

[ccz145a]

master first, a broken booster would not cause that.

[Captkaos]

Did you open the system when you replaced the brakes?  If so, did you bleed the system after you were done?  If the reservior was empty in the master cylinder you are probably will have to bleed it.
What changed before and after the brakes?  Was it working before?  Its not the booster.

[VileZambonie]

Did you pull the drums and look at the wheel cylinders? Make sure they are not wet?

[gold84gmc]

well i replaced the wheel cylinders a while ago. and they dont appear to be leaking. when i checked the resevoir as i stated above, it was not low enough for air to get into the lines, was only about half empty. and i havnt touched the front brakes yet.

[VileZambonie]

If you are POSITIVE you have no external leaks (wheel cylinder, caliper, hose, line etc) then replace the master cylinder.