If you can get your hands on a stethescope listen for gurgling in the
booster. If your getting this sound there is a lot of fluid inside of the
booster and it is probably rusted badly internally. Ask me how I know! I
had the same problem that your describing and after bleeding the brakes
several times I finally decided to listen to the booster to see if there
was an internal problem. Sure enough, I needed to replace the booster.
Like it or not the dash has to come out in order to get to everything. It
makes doing the job a whole lot more pleasant, than trying to get big
hands in little spaces and losing parts in a maze of wires. Besides, while
your in there you can clean up any mess left behind by unwanted guests.