My brakes started feeling a little soft and then got to the point where the pedal went to the floor with no stopping power. So I started looking into the brake system and found some corrosion issues and replaced some lines, replaced the MS (twice) and did other troubleshooting. Pressure bleed the brakes multiple times, used my Scanner on the ABS, bench bled the new MC and isolated parts of the system to try and solve the problem. When the car is off and press the pedal a couple of times, the brake is firm and hard. However, upon starting the Blazer, the peddle goes soft and travels to the floor.

I have also capped off the MC ports with plugs and got the same scenario: firm brake when off, but pedal goes soft when car turned on and boost applied. I have checked the booster check valve (good), booster line (good) and no visible leaks at MS or Booster. I am getting no rough idle or Check Engine or ABS lights.

It appears to be pointing to the Booster, but these symptoms aren't making sense (a bad Booster or not) - can anyone help or point me in a better direction? Is it time to replace the Booster?

This is a 2000 Blazer LT with 2WD 4.3L and has over 200K miles.

 

Maybe try pinching the hose from the booster to the intake with vise-grips. If the brake pedal then acts normal, this might indicate a leaky brake booster.

 

If the brake booster has a leak, I would guess it could also affect the engine due to there being a vacuum leak. Is your engine working fine or change how it runs when you depress the brake?

 

Strange - I put the wheels back on, dropped it off the jacks and was backing vehicle out of shed - and I now have brakes! I believe the first "new" MC was bad, but the second is working. I have a good firm brake.