1989 328 GTS "Brake Failure" What Is the Deal On This Fault Issue And Lack of Braking?

Yes, in the 89 cars hydraulic pressure is provided via a pump and accumulator. The pump is called upon to keep the accumulator pressurized via a pressure regulator. There are also several fuse-protected relays that control the functioning of the pump. All but one of them are located next to the brake fluid reservoir under the fiberglass cover in the front compartment. The other one is mounted to the passenger side footrest on the back.

The pump, regulator and accumulator are historically quite reliable. It's usually a faulty relay that causes issues.

Can you describe in more detail what happens when the brake pedal is applied? You said hard as a rock and not working correctly, but it would help to understand exactly what that means. For instance, does it require extraordinary effort on the brake pedal to get the car to stop? Or is it just that the pedal feels wooden and doesn't want to move?

Could be brake pads worn past their useful life and causing a pad sensor to trigger the warning light.

A couple years back I spent weeks learning all the details of the various parts of the 1989 brake system, including the ABS bits (ECU, wheel speed sensors, fluid distribution block). I was troubleshooting an issue where the pump wasn't pressurizing the accumulator, and it turned out to be a faulty relay located behind the passenger footrest. Here's the thread where I described the problem and solution, in case you find some of it useful.

 

Jerry, I forgot to give you a link to the thread I was referring to. Here - Help with simple relay operation | FerrariChat . The good news is the solution was a cheap relay. It just took a long time to determine that.

If it takes a gorilla to stop the car, then the accumulator isn't providing the braking assistance like a vacuum booster would. Most likely a faulty relay. If all is working, you will hear the pump run when you first turn the key to position 2 ("on") before cranking. It takes about 30 seconds for the accumulator to fully pressurize, then the pump will stop. Also, the cascade of relays is designed to temporarily turn the pump off while the engine is turning over, to allow for more juice to the starter during that critical time. As you drive around and use the brakes, the pressure dissipates in the accumulator, and at a certain point the pressure regulator tells the relays to turn the pump on again to maintain boost pressure in the accumulator. You never notice it running when the engine is on though.

I documented how all the relays and stuff works in a spreadsheet. I'll upload it here in a few minutes so anyone can reference it if/when they need to troubleshoot a failure.

 

Attached is the Fig 9 (Brakes/ABS) diagram with left and right halves merged into a single page.
Also attached are the notes on brake system function.