My peddle goes to the floor. Fluid isn't leaking any where. New Motorcraft master cylinder. ( listened to a friend) brakes blead very well ask my wallet. So I have brakes if I pump the peddle but it still sinks to the floor. My guess is brake booster. Probably should mention 5.4 motor. Pretty good vacuum at the booster. The bottom third of canister/cylinder is rusted pretty good. Could there be pin holes in it causing it to leak down? No easy fix if this is the case except replace?

 

Could be a number of things but if the booster is rusted bad, I'd replace it just to have piece of mind that it won't fail in an emergency situation.
Recheck the master cylinder to make sure the fluid isn't bypassing the seals, I've had this happen before, brand new out of the box.
Also did you make sure the ABS module was bled correctly? If you lost fluid originally and it went dry, you could have air in the module..

 

ok I'll start by saying im a pretty good parts changer. Ive worked on my own stuff all my life. Im no mechanic by any stretch of yours or mine imagination. I do ok most of the time not paying someone $80 to $100 an hour to work on my stuff. This being said fluid bypassing the seals. Not sure how i would check this. I get good pressure when im bleeding the brakes and the peddle doesnt go to the floor with the truck off. Unless of course when bleeder is open. Lol. On the abs module I never lost fluid originally. I got to the job site and went to turn around to park and peddle went to floor. I figured I blew a line. Not fluid lose at all and for the next two days I drove it waiting on new master cylinder. Have to work and breaks are optional right? Ha ha. When I changed the master cylinder I don't believe I lost hardly any fluid out of the 2 lines that connect to it. My abs light has been on for a few months but I figured it was a ground some where . It sat for about a year and a half and have been chasing grounds and issues every where. Could you give me a little direction on bleeding the abs module as ill do anything that helps me narrow this down. Thank you for your response and hope this wasn't to long winded. It's pouring out right now and I'm home not working because of it but can't really get out and work on the truck either.

 

Did you bench bleed the master cylinder before you installed it?
as for bleeding the ABS module, if you didn't run the system dry I doubt that's your problem but from what I understand the OBD has to be hooked up in the shop for a bleed procedure....electronics, got to love them.

 

Yes bench bleed master cylinder

 

I will add it is doing the exact same thing with new master cylinder as the original master cylinder

 

If you pump pedal and have breaks, but it'll still go to the floor on regular breaking, I'm still going with either the new MC was not bled out properly or it's a bad MC right out of the box. I know it's a PIA, but if it were me, I'd remove the MC and re-bleed it real well just for piece of mind..

Here's a couple of videos for you to watch, They might help you diagnose the problem

This one isn't the best quality but it's something you can try.

 

thank you I take a look