How do you tell if the power brake booster is bad on a 1997 ford taurus

proportioning valve

Brake booster vacuum hose , inspect that, if the hose is good then brake booster has went bad.

vacuum leaks can make booster loose power assist. if pedal intermediately hard to push the booster might be bad. if the pedal goes to the floor and you dont have brakes might be the master cyl

Usually when the brake booster goes bad, when driving and trying to stop, the pedal is VERY hard to depress and the vehicle will not stop easily so don't drive fast, you will not be able to stop in short distances.

sounds like the brake booster failed ! or the vacuum line to booster. also check for a bad check valve at the booster hose where it goes into booster.

If your not finding leaks at the wheels remove the master cylinder from the power booster and look for a leak there. You don't need to disconnect it, just the two nuts holding it on and then very easily move it away far enough to see.

hello there:
you may have over looked the vacuum line for the cruise control that hooks up to or close to brake booster hope this helps you out..sincerely D.Jones

1. Step 1 Mark the brake lines coming into and out of the master cylinder using masking tape and permanent marker.
   
2. Step 2 Orient the tube wrench so that it grabs the end of the nut by first sliding it over the brake line.
   
3. Step 3 Pull the brake lines out of the master cylinder.
   
4. Step 4 Unbolt the two bolts that hold the master cylinder to the brake booster.
   
5. Step 5 Slide the master cylinder off the brake booster.
   
6. Step 6 Remove the bolts that hold the brake booster to the firewall.
   
7. Step 7 Remove the cotter pin from the pin holding the back of the brake pedal to the master cylinder.
   
8. Step 8 Slide the brake booster to brake pedal pin out of the mounting bracket.
   
9. Step 9 Pull the brake booster off the firewall.
   
10. Step 10 Install the new brake booster. Installation is the reverse of removal.
    

your brake booster went out, or you have a major vacuum leak, provided the pedal is still firm when you press it. It'll usually feel like stepping on a rock, but the brakes will still work to a small degree. The booster is more likely. If the pedal goes to the floor, you have air in the system, a blown brake hose, wheel cylinder, or caliper.

Do the brake lamps work? If they don't check for blown fuses. The brake lamp switch may have shorted when the booster was replaced.