I replaced the brake master cylinder on my f250 sd because the pedal was leaving me at red lites. still got same problem .

 

Did the level in the master cyl go down since you last filled it? There is a good possibility one of the hydraulic components is leaking under pressure . Depending on where the truck is used, salt covered roads, constant wet environment etc., the brake lines rust and start to leak through a small pinhole. A thorough inspection of all the wheels and brake lines may reveal a leak. If that is the case you won't have any trouble finding the wet spot from the leaking brake fluid.
And as always,,,,,,the master cyl you installed could be defective. You did bench bleed it before installation, right ?

 

thanks hanky. I'm in w tn. not losing any fluid,my pedal creeps down at a red lite, like it's bypassing in the master cylinder. same as old m/c .I am retired and have always tried to do my own mech.work .bench bleed. any help would be appreciated.

 

Even if you didn't bench bleed the master cyl before installation if there is any air in the lines you would get a soft pedal or no pedal at all, but it would not leak down as is happening now. If you can try google. "Bench Bleeding a master cyl". A picture is worth many words and is more pleasant to watch that me trying to describe it.

 

Do you have drum breaks on the rear? If so check the adjustment of the shoes. If the shoes are too far away from the drum you can get similar symptoms.
In any case do take a good look at your system. Breaks are kind of important.
Regards,
Eric