Just starting having this problem on my 2001 Expy XLT sport 5.4L. The brake pedal slowly sinks almost all the way to the floor when stopped and pressure is applied to the pedal. Took it to the mechanic who said it was the master cylinder. Two master cylinders later (all in one day) with bleeding of the lines each time ($200.00 part and labor), and replacement of all brake pads (another $150.00) and the problem is still not 100% fixed. Now there's pressure buildup in the brake pedal when pressure is initially applied, but at least 80% of the time the pedal still slowly begins to sink to the floor. What should I be looking at as the likely cause of this problem before trying my luck with another mechanic who may not be any better than the one I have tried, or worse yet, going to the Ford stealership? Will be travelling from NY to PA this weekend and need to get this problem fixed!

 

I've seen batches of rebuilt master cylinders be no good. Was it a new MC or a rebuilt one?

Are you sure there are no leaks in any of the brake lines? It doesn't have to be a big one.

 

Mechanic claims MC was brand new and that there were no leaks in the line. Is it possible he did not get all the air out? Don't know if this additional info helps, but I do not recall seeing him bench bleed the MC before installing it. However, he did use the pedal pump method.

 

It has to be bench bled, you cannot bleed the MC after it is installed. The "pedal pump method" does not allow the plunger within the MC to travel the full length of the bore.

 

Took the truck to another mechanic today. He took off the MC and bench bled it then reinstalled it and bled the system but the problem was still there. He then replaced the power booster but the problem was still there. Finally, he ordered a new MC and installed it after bench bleeding, etc. still has the problem. He hooked up a scanner but got no codes, the CEL is not lit and neither is the brake light on the dash. He thinks it might be the ABS. What do you fine folks think? All the MCs tried so far were aftermarket and were from three different stores, do you think that the manufacturer's brand should be used instead of aftermarket brands? Any help is welcomed. I'm thinking of going to the Ford dealership on Monday for a diagnosis since 2 independent mechanics can't seem to find the problem.

 

It could be a leaking valve in the ABS unit, but I would think you'd get a code.