ol '90 6000 brake question..

my grandfather gave us this car for a spare, and now we need it.. but one thing makes it pretty difficult, especially on wet ground.. the brakes seem to be all-or-nothing.. initially with it topped off, the slightest tap would lock the tires up.. rather unexpectedly on dry ground even.. I siphoned out some of the fluid and now it still behaves the same only it happens further down the brake travel... no or little effective braking then wham! I'm pretty used to a nice smooth brake pressure curve, this isn't something I'm getting along with.. my stuff always flying from the seat to the floor..lol (gas is pretty touchy too but I can deal with that more).. any way you folks can help point me to check it out? this car was granpa driven and maintained,so in otherwise top-notch mechanical condition... strong as an oxe, freebie or not it's worth a little work to me..