Soo here's the scoop 2001 xl 4.6l with all disc brakes I bought the truck and all the brakes were good pedal was nice and solid brakes worked great then I started lookin and to notice the previous owner had the drivers side rear brake pinched off so I proceeded to fix it bleed all the brakes and now the pedal won't get stiff at the top it only goes all the way to the floor before it gets stiff but when the truck isn't running it gets stiff at the top like it should

So I then preceded to install a new master cylinder and still the same thing

Any thoughts maybe booster antilock brake system??? Also I'm not 100% too what the big black thing that looks like a master cylinder that the lines run to from the master cylinder and then split to three after it

Help is very much appreciated

Note it also has cruise control

 

Does your pedal fade to the floor and then get firm?

https://www.f150forum.com/f6/brake-p...-150-a-157374/

I had to buy an new ABS module. It's probably the HCU.

 

Ya that what I was thinking I'm going to try and rebleed the whole system again to make sure

How much that run you?

 

I got lucky and found a brand new one on eBay for about $200.

 

When you had the vehicle shut off would the brakes pump up to the top? Prior to chi replacement

 

That is not pumping up to the top. You're just losing the benefit of the brake booster. I think that's a red herring.

 

Ya that's what I figured but weird how once more pressure is applied (booster) the system basically weakens it almost seems still a possibility there's air in the lines or master ..... Last resort is abs hcu but hopefully not😕