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Unsolved Brake mystery with 1995 Ford E350 van, brake pedal goes to floor

After a recent front brake caliper replacement, the brakes work great except that when moderate pressure is continually applied to the brake pedal, the pedal goes all the way to the floor. While driving down the road, when the brake pedal is depressed, the brakes engage right where you expect them to, and the van stops just fine, even if the stop is extreme. My question is, please explain what is going on? There is no air in the brake lines. The front calipers have been replaced twice, the back brake cylinders replaced, the master cylinder replaced, the RABS valve replaced, all by a certified shop trying to address this issue.

I spoke with the service manager at a local Ford dealer in Seattle and he was aware of the problem and said that there is no solution. He said the brakes will work fine. He couldn’t explain to me however, why this happens. If the van is not running, the brake pedal holds firm, so I assume the problem has something to do with either the ABS system or the power assist, but I can’t say for sure. I can live with this since the van stops great. However, I would sleep better if someone could explain to me what is mechanically going on that allows the brake pedal to go to the floor even though the brakes are fully engaged after the pedal is depressed say a third to a half way down to the floor (where you would expect).

Sounds like a classic case of a bad master cylinder, you may have gotten a faulty one.

The shop changed the master cylinder twice thinking the same thing. No difference!

Ford apparently acknowledges the problem, I just want to know the how/why of it. Once the brakes are fully engaged, I am not understanding what allows the brake pedal to continue traveling to the floor. I understand these things quite well but this one is a puzzler!! No air in the lines, no leaks, brakes work fantastic. It’s just I don’t understand what allows the pedal to continue travel to the floor.

Something is definitely not working. If it isn’t the master cylinder, it might be the brake booster has sprung a leak and introducing play in the system. This could explain why it is different if the engine is running or not, as the booster is affected by applied engine vacuum.

Try driving with the RABS valve unplugged. RABS valves can dump on every brake application if there is a chipped tooth on the rear axle exciter ring, electrical induction into the RABS sensor circuit from high-current flashing lights such those on ambulances and the like. I say that because I’ve seen it once where the ambulance builder wire-tied the wig-wag brake light harness to the RABS sensor harness from front to back and the RABS system paid attention to the frequency of the brake light flashes rather than the exciter ring and interpreted it as a brake lock-up. This kind of thing triggers no codes.