I have had a strange brake problem on my 2004 Expedition for some time now. Previously I decided to change the master cylinder, but it happened again. Here is what happens - when the brake pedal is pushed very rapidly - it goes to the floor and then is very hyper sensitive. It brakes really hard with very little pedal. Then seconds later it is normal and functions properly. I checked the master cylinder and the brake fluid was a little low, but the symptom was the same as that prior to the master cylinder change. Again, only with very rapid pedal movement. No evidence of leakage on the concrete .....

Any ideas on where to look next?

 

I would suspect the valves in the ABS unit are internally leaking.

 

Did you bleed the master cylinder when your replaced it? It could also be air trapped in the ABS

 

Yes I did at the cylinder and all 4 wheels.

 

What you're describing is a safety feature introduced with the 2003 model year. To quote a website..."A new feature called Brake Assist further reduces braking distances for the new 2003 Expedition. It constantly monitors the speed and distance of brake pedal travel. If the system determines from a sudden application of the brake that the driver intends to make a rapid stop even if the pedal isn't fully depressed - it boosts hydraulic braking pressure automatically."

http://media.ford.com/press_kits_det...ection_id=2873

 

Thanks for that, but I can tell you - it doesn't help. Something has to be wrong with this feature on my Expedition. When it happens, the braking becomes so sensitive it is almost uncontrollable. It is very soft and very little pedal movement almost locks up the brakes. Basically, you cannot control the brakes.

 

While I can't drive your truck and say for sure that what you're feeling is or isn't normal, I can say that it is alarming when the assist engages and you're not expecting it.

Here's a tech tip from another discussion thread on this...
Tech Tip: Expedition/Navigator Unintended Brake Assist

From this thread...
https://www.ford-trucks.com/forums/1...e-problem.html

 

Thanks for the help!! That last thread describes the exact same problem.