Brakes sink all the way to the floor when I push down

Hi,

First, make sure the brake fluid is full in the master cylinder reservoir under the hood. If it is, check to make sure there is nothing leaking under the vehicle.

If the reservoir isn't low and there are no leaks, 99% of the time the master cylinder has failed causing this. Check out this link to see if it mirrors what you experience.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

If you determine the master cylinder to be bad, and I suspect it is, here is a link that explains how to replace one:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Here are the directions specific to your vehicle. It includes bleeding the system and removal and replacement of the brake reservoir if needed.

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2002 Mercury Sable V6-3.0L DOHC VIN S
Brake Master Cylinder
Vehicle Brakes and Traction Control Hydraulic System Brake Master Cylinder Service and Repair Procedures Brake Master Cylinder
BRAKE MASTER CYLINDER
Brake Master Cylinder

Removal and Installation

WARNING: Use of any other than approved DOT 3 or DOT 4 brake fluid will cause permanent damage to brake components and will render the brakes inoperative.

WARNING: Brake fluid contains polyglycol ethers and polyglycols. Avoid contact with eyes. Wash hands thoroughly after handling. If brake fluid contacts eyes, flush eyes with running water for 15 minutes. Get medical attention if irritation persists. If taken internally, drink water and induce vomiting. Get medical attention immediately.

CAUTION: Brake fluid is harmful to painted or plastic surfaces. If brake fluid is spilled onto a painted or plastic surface, immediately wash it with water.

1. Disconnect the battery ground cable.
2. Remove the air cleaner outlet pipe.

pic 1

3. Disconnect the brake fluid level sensor electrical connector.

pic 2

4. Loosen the fittings and disconnect the brake tubes.

pic 3

5. Remove the bolts and the brake master cylinder.

6. NOTE: If a new master cylinder is being installed, the new master cylinder must be bench bled.

To install, reverse the removal procedure.
7. Bleed the brake system.

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Reservoir

2002 Mercury Sable V6-3.0L DOHC VIN S
Brake Fluid Reservoir
Vehicle Brakes and Traction Control Hydraulic System Brake Master Cylinder Service and Repair Procedures Brake Fluid Reservoir
BRAKE FLUID RESERVOIR
Brake Fluid Reservoir

Removal and Installation

WARNING: Use of any other than approved DOT 3 or DOT 4 brake fluid will cause permanent damage to brake components and will render the brakes inoperative.

WARNING: Brake fluid contains polyglycol ethers and polyglycols. Avoid contact with eyes. Wash hands thoroughly after handling. If brake fluid contacts eyes, Hush eyes with running water for 15 minutes. Get medical attention if irritation persists. If taken internally, drink water and induce vomiting. Get medical attention immediately.

CAUTION: Brake fluid is harmful to painted and plastic surfaces. If brake fluid is spilled onto a painted or plastic surface, immediately wash it with water.

pic 4

1. Disconnect the brake fluid level sensor electrical connector.

pic 5

2. Use a suitable suction device to drain the brake master cylinder reservoir.
3. Using a screwdriver, carefully pry up on the brake master cylinder reservoir and remove.

4. NOTE: Whenever installing a new master cylinder reservoir, install new grommets.

NOTE: Lubricate the grommets using Ford High Performance DOT 3 Motor Vehicle Brake Fluid C6AZ-19542-AB or equivalent meeting Ford specification ESA-M6C25-A before installing the grommets into the master cylinder, and before installing the reservoir. If DOT 3 is not available, use Ford High Performance DOT 4 Brake Fluid or equivalent meeting Ford specification SAE-J-1704.

To install, reverse the removal procedure.
5. Fill the reservoir with Ford High Performance DOT 3 Motor Vehicle Brake Fluid C6AZ-19542-AB or equivalent meeting Ford specification ESA-M6C25-A. If DOT 3 is not available, use Ford High Performance DOT 4 Brake Fluid or equivalent meeting Ford specification SAE-J-1704.

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2002 Mercury Sable V6-3.0L DOHC VIN S
Master Cylinder Priming - In-Vehicle or Bench
Vehicle Brakes and Traction Control Hydraulic System Brake Bleeding Service and Repair Procedures Master Cylinder Priming - In-Vehicle or Bench
MASTER CYLINDER PRIMING - IN-VEHICLE OR BENCH

Master Cylinder Priming - In-Vehicle or Bench

pic 6

1. For in-vehicle priming, disconnect the brake lines from the brake master cylinder .

pic 7

2. For bench priming, mount the brake master cylinder in a vise and install short brake tubes with the ends bent into the brake master cylinder reservoir.
Fill the brake master cylinder reservoir with High Performance DOT 3 Brake Fluid C6AZ- 19542-AB or DOT 3 equivalent meeting Ford specification ESA-M6C25-A (United States and Canada only). If DOT 3 is not available, use Ford High Performance DOT 4 Brake Fluid meeting Ford specification SAE J1704.

3. Slowly depress the primary piston or, for in-vehicle priming, have an assistant slowly pump the brake pedal until clear brake fluid flows from both brake tubes with no air bubbles.
4. If necessary, install the brake master cylinder into the vehicle.

pic 8

5. Remove the short brake tubes and install the master cylinder outlet tubes.

6. NOTE: Refill the brake master cylinder reservoir as necessary.

Have an assistant depress and hold the brake pedal.

pic 9

7. Loosen the brake outlet tubes at the master cylinder one at a time.

pic 10

8. Tighten the outlet tubes.
9. Repeat the above three steps until no air bubbles are seen in the expelled brake fluid.
10. Bleed the brake system.

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Let me know if you have other questions or need help.

Note that I added the reservoir removal and replacement because on some new master cylinders, you have to reuse the old one. Also, the bleeding procedure listed above is easier if you do it before you install the part. If you don't have a vice to place it in, it will require more than one person to do it and isn't as easy.
Take care,
Joe