We have a conversion van with ABS brakes. While returning from Georgia this week, The brake pedal started going to the floor although it never did go all the way, and the ABS light came on. Today I replaced the master cylinder and I bled the thing on my bench. After installing it I noticed that without the motor running the pedal is full and hard, but as soon as I start the engine it goes soft and goes to the floor. The ABS light is still on too. Has anyone else had this problem? if so please let me know how it was resolved.

 

Welcome to FTE. If you haven't figured the problem out yet there is a brake forum down the page. The gurus there might know more. Good luck with it.

 

You have a broken brake line at the rear of the vehicle. The REAR brakes are what determines how the brakes feel. You need to get under it and look at all the lines from the proportioning valve back. It could even be a bad wheel cylinder (if you have drum brakes) or a bad caliper (if you have rear discs). My' 91 merc does this same thing, no abs though. It's just air in the lines.. I'm too lazy to bleed it right now as the brakes work fine other than the squishy pedal.

 

I found bleeding an ABS vehicle is difficult, they eleminate the brakes from locking by limiting the pressure, so they do similar bleeding, making it difficult, I'd like a vacuum tool to bleed them.