So I've done a few hours of digging on forums and ran a whole list of tests and still can not figure out what in the world I have going on. So I personally have a 2012 F150 3.5L 2WD XL w/ about 200,000mi. I have the front of the truck up on stands because I had to put new front struts on it. (Shot a chunk of coil spring out onto the driveway the other day) guess it didn't want that anymore. But any way. I noticed that the friction material on the outer brake pad on the L.F. wheel was just about totally gone. O.K. So I check that the rotor can turn. Its dragging/locked up pretty good. I then notice that the R.F. wheel is dragging/not fully releasing. But the pads on this side don't look to pad. So I change out the pads at both front wheels with a set I had at the shop. Do a vacuum bleed at both front calipers, and then do a manual bleed at both front calipers. The L.F. caliper was not bleeding well. Go to test the brakes on the stands and both L.F. and R.F. brakes are not releasing. I was eventually able to get the R.F. to release a little bit, but it still dragged pretty good. I pulled the L.F. caliper apart to inspect the internals. Everything looked good. But I decided to get two new front calipers/brackets anyway. So I install those with the newer pads I had at the shop. Vacuum bleed them, then manually bleed them. The L.F. still just doesn't bleed like it should. The vacuum wasn't pulling much fluid, and even the manual bleed at the L.F. wasn't moving much fluid. Go through that. Go to test the brakes again (with the truck off) R.F. is releasing enough for me to be o.k. with it, but L.F. is rock solid, not releasing. Then we start the truck test the brakes on the stands and now the L.F. and R.F. are both locked up. I'm assuming because I then had the power assist of the brake booster. So for my next test I press the brake pedal, both calipers are locked up, I crack the bleeder valve on each caliper still locked up, I loosen the rubber feed line hose to the caliper on each side, still locked up. Even though there's no fluid pressure at them. So then I'm real confused. The calipers should not be seazing up. They are new fomoco remans. So after that I took the L.F. caliper off the bracket and attempted to push the pistons back in one at a time with the bleeder screwed in and the rubber feed line hooked up. And it struggled to get them pushed back in. It would just push the piston next to it out. So that has me thinking maybe a bad caliper hose, but still doesn't explain why the pressure does not release when I open the bleeder valve and the feed hose. Next I took both calipers totally off the car removed the bleed screws removed the feed line hoses, and attempted to push the pistons back in. It felt like all the pistons had a little bit off resistance right when I started clamping on them, but would then push in nice and smooth. almost like a cocked piston (but all four?) Any who I'm out of ideas. Next thing I'm gunna try is getting the rotors turned. They have some pitting and a little waving in them, brand new pads, and new brake hoses on both sides. I hate throwing parts at things but I'm truly stumped. And I do this for a living. If anybody made it through this whole post and has ideas let me know. And yes I've done all of the simple things flushed all new fluid through it, checked calipers and brackets parallel to rotor, checked all hardware (pins pad slides etc).

 

I picked a few words out of your pile. Pads on backward and/or service bleed come to mind.

 

I have heard of lines failing internally without much sign from the outside - I would check that those rubber lines aren't preventing the fluid from going back when you let off the brakes.

I haven't ever really tested mine like that - dont these trucks have some kind of stop assist for when you're on a hill? perhaps with the front up it thinks you're on a hill and is applying brakes, Im not sure how to verify or disable that

 

Pads are on correct, and ive vac bled and manually bled a ton of fluid through them.

 

So I put new rubber lines on both fronts that go to the calipers. The R.F. was a little restricted and the L.F. was extremely restricted. I could not even blow through that one. Also had the rotors turned so that there good and true, and put all new front pads in as well. Did a vac bleed and manual bleed again, and now the R.F. is working good. Does not hang up and releases as it should, and the L.F. releases but still has a decent bit of drag. Also noticed that when I have the truck running and press the brakes that the pedal is kind of soft and sinks slowly. Looks like its getting a master cylinder next.

 

This sounds like a blocked hydraulic line. You should have opened the bleed screw to see if the piston moved independently of the other side. Fluid should squirt from the bleed hole. That would confirm that the pistons move well and the bleed hole is clear.

I would remove the calipers and see how flow is through the lines alone. And examine the port in to the caliper closely. You said that you've pulled a ton through but above you said that the left side is not flowing well.

 

The L.F. caliper hose was totally blocked, and the R.F. hose was partially blocked. Replaced those and now the issue is the L.F. caliper still isn't fully releasing and the peddle is more mushy then it was before and sinking to the floor.

 

If you pump the pedal does it stop dropping to the floor? If not it sounds like a bad master cylinder. If it does pump up it's air in the lines.

Not uncommon to damage the master cylinder seals when the piston travels over parts of the bore that have not been used before, after brake work.

 

So latest update. I went out this morning and looked at the rear brakes just to see if there were any issues back there. Plus I wanted to take the parking brake shoes and assemblies out because they were shot and I don't use them. Everything looked o.k. back there. I re greased all the slides and pad hardware and put them back together. Also did a vac bleed on both rears as well "why not I guess". and noticed that the L.R. wasnt pulling as much fluid as the R.R. So I may have a hose issue at the R.R. as well. Caliper releases just fine though. Then I moved back up to the front. Did a vac bleed again on those. Same situation R.F. caliper releases perfect. No drag at all. But the L.F. caliper does not release all the way and drags pretty good. One thing I did notice that may tell the story is that if I crack the bleeder at the L.F. caliper it still drags. But when I hook up the vac bleeder and pull vacuum and then open the bleeder. The caliper acts exactly as it should and releases. So its like the fluid isn't being pulled back at all at that L.F. caliper. With that being said one would think the issues is in the master cylinder but if that were the case you would think the issue would be at both front calipers. Also the power brake booster vacuum pump has been making noises for probably close to a year, but from my understanding that would not be pulling vacuum on the brake hydraulic system. Pedal still sinks down to the floor pretty good but I took it out for a test drive and the brakes work just fine. Only thing that is weird is that the pedal slowly sinks a little when you come to a complete stop. Never losses total brake pressure though and stays holding. During the test drive I also went out in some gravel to activate the abs and then came home and did a re bleed on the fronts. Still same problem with the L.F. caliper dragging.

 

When the truck is running and I pump the pedal it does build up and get a little stiffer, but still eventually starts to sink.