2005 f150 xl 5.4 4x4, My front driver side caliper locked up on my rotor yesterday on the way home from work. bought new caliper and replaced it. bled the brake and still felt as if there was air in the lines(brake pedal to the floor/spongey) So this morning i took it in to a mobile one express and they bled the brakes 3 more times and then the guy says hes not sure but he thinks my master cylinder is shot. He didnt seem certain and i dont wanna spend the money if thats not corrct. Does that sound accurate?

 

Are the bolts holding the master cylinder to the booster tight?

 

yep

 

The master has a piston in it and when the seal on the piston goes the brake pedal will slowly sink to the floor. Was it spongy or does it sink to the floor.
If it hold pressure then I believe that the master cylinder is not your problem.
Did you let the master cylinder get low while changing the caliper?

 

It goes all the way to the floor the first time and then after pump-ing it it slowly comes up but doesnt return to where it should be. The master cylinder never got to low when changing the caliper. I made sure to continue and top off the resovoir while doing the caliper.

 

Well, at this point I would visually inspect every inch of your brake system.
If your pedal holds pressure, then I still think that it is a fluid/air problem.
Take a good close look and pay close attention to your rubber hoses. Make sure their are no issues with the lines and if all looks ok there, I would bleed the entire system. How old is the brake fluid in the system?
When bleeding the entire system start with the farthest wheel. So you
would bleed RR,LR,RF,LF.

Anyone else have any thoughts??

 

keep bleeding it till you get all the air out.

 

The brake fluid is new. It has been bled a total of 5 times. Theres a possibility there could still be air in the line?

 

Possibly an issue with back brakes?
Here's something to read..

 

I would take the master cylinder off and adjust the actuator rod out a few turns to bring the pedal stop to center stroke. Adjust your drum brakes externally with a screwdriver so that you know the actuating distance is right. If you need info on how to do this, just ask.