I need you to test something for me. First, confirm the brake fluid is full in the master cylinder reservoir. Next, start the engine and apply the brake. Hold steady pressure on the brake pedal. Does the pedal slowly go to the floor? If it does, chances are the master cylinder is bad. Take a look through this link:
Also, if this happens, let me know and I'll provide directions for replacement.
If the brake fluid was low or empty in the reservoir, then check for evidence of leaks under the vehicle, at each wheel, and near the master cylinder.
I attached a pic below of what the master cylinder looks like and where the full level.
Last, if the pedal doesn't go to the floor and there is no fluid loss, the first thing I suggest is to readjust the rear drum brakes. Here are the directions. The second pic below correlates with these directions.
1997 Buick LeSabre V6-3.8L VIN K
Vehicle Brakes and Traction Control Drum Brake System Adjustments Drum Brakes
Brake Component Views
1. Raise vehicle and suitably support.
2. Mark relationship of wheel to wheel studs in bearing flange.
3. Rear wheels and tires.
4. Mark relationship of drum to wheel studs in bearing flange.
5. Brake drum.
6. Make sure lever stops on both park brake levers are against edges of shoe webs on park brake shoes. If park brake cable is holding stops off edge of shoe web, loosen park brake cable adjustment.
7. Measure the inside diameter (ID) of the drum using J 21177-A or J 22364-01.
8. Turning star wheel on adjusting screw assembly, adjust shoe and lining diameter to be 1.27mm (0.050 inch) less than the drum ID.
9. Install drum and wheel, aligning marks made at removal.
10. Repeat steps 2 through 9 to adjust the other rear brake position.
11. Torque wheel nuts Tighten 140 Nm (100 inch lbs.).
12. Lower vehicle.
13. Apply and release service brake pedal 30 to 35 times with normal pedal force. Pause about one second between pedal applications.
Let me know what you find or if you have other questions.
Images (Click to enlarge)
Thursday, November 28th, 2019 AT 7:35 PM