The brake system on an 86/87 GN (some 85's too I think) is called a PowerMaster. It has an electric pump (under the master cylinder) that builds up pressure which is stored in the black ball (accumulator) This pressure is used to assist braking (power brakes)
There are a couple things that go wrong.
Bad switch (gray or black thing on top of the booster) which can make the motor run too long which burns out the motor or blows the fuse.
internal leaks or checkvalve causing the pump to cycle too often
bad accumulator which causes a momentary hard pedal when hitting the brakes.
bad pump motor (all sorts of bad stuff)
leaks (even more bad stuff)
Now, most parts are not available.
Remanufactured complete units are available, and you can find used stuff on here.
As a backup plan, you can switch the car to vaccuum brakes, and keep the PowerMonster in a bag to rebuild "someday"
Best of luck
Bob