I have already checked the brake fluid level. 4 times. It’s full.
I had the brake rotors machined about 20k miles ago. I’m not getting any of the marker warnings, vibration in the pedal and no squeal.
Could it be the master cylinder? I know they can leak or fail completely. Can they get weak and cause this problem?
My brakes work smoothly and quietly. I don’t like how far the pedal goes towards the floor.
Possibly air in the brake lines. Master cylinder failure might make the pedal feel spongy.
Check the Vacuum booster for the Master Cylinder. It’s a large round thing with a rubber hose attached to it and the engine. If the diaphragm ruptured, your pedal will feel wonky. This, of course assumes Power Brakes.
As CookingWithGas said, air in the brake lines, or even a leak, although with a leak, you would be losing brake fluid.
How long has this been going on? Have you had your brakes recently serviced? If it is a recent development and haven’t had any work done lately, I would suspect a leak, you will lose fluid if that is true. If you just had some work done, it is quite possible they did not bleed the system properly. How old is the car and when was the last time you had your brakes serviced?
As you may know, your brake system is actually two brake systems, one works the front left and right rear brakes and the other the front right and left rear. This is so that even with a leak in one of the lines, you will still have brakes. While you don’t notice a loss in performance, you are not stopping as well as you should be. You need to get it looked at as soon as possible.
I’m going by the tire & brake shop Monday.
I know new rotors and pads will eventually be needed. I had the rotors visually checked after my last tire rotation. They weren’t too thin.
I’ll ask them about the master cylinder and vacuum assist.
Sounds like the master cylinder. Hell, replacing the master cylinder on my old Ford Ranger is practically routine maintenance. Which reminds me…
the first step for a DIY’er would be to bleed the brakes, almost no cost, and not much time. The first step for the non DIY’er would be to bring it in.
Let us know what they say. I know I’m curious and I’m sure others are as well. Good Luck!
I’ll post afterwards.
I’m hoping it’s something relatively inexpensive.
Ford OEM rotors on a E150 are expensive. I wouldn’t use after market. Brakes are too important.
Worn rotors won’t cause the pedal to go to the floor.
What CookingWithGas said. You can YouTube how to bleed the line. It’s free and easy.
An interaction with my father roughly 25 years ago:
Me: Hey dad, the brakes on the T-Bird are kinda mushy. Mind checking them out?
Dad: They’re fine. Just pump them if they get like that.
Me: Okaaaay…
…time passes…
Me: The brakes are really mushy now. Like, they went to the floor once. I started rolling when I was at a stoplight and had to shift into park.
Dad [dismissive tone]: All right, I’ll check them out when I get a chance. But they were fine when I drove it the last time.
…time passes…
Me: The brakes are gone, dad. I had to make a free right turn at a red light because the car wouldn’t stop. And had to half drive into a ditch to avoid a car when I was going down a hill.
Dad: Fine, fine, I’ll check it out.
…a short time later…
Dad: How the hell did you get this car back in one piece? The master cylinder is cracked in half!
In short, yes: it can be the master cylinder.
Ditto for a worn master cylinder.
One thing I also heard- the brake fluid could absorb water in a humid environment for assorted reasons. The symptom of this (I think) is that the brakes are fine until you’ve driven a while, then they get mushy because the braking creates heat which boils the water and now you have steam bubbles in the line, which is like air in the line. But typically this is why big semi brakes fail on long hills.
Another reason why changing brake fluid every so often is a good idea in some environments.
The master cylinder is faulty, leaking internally. Replace it.
There’s no point to bleeding the brakes. It’s extremely unlikely there’s air in the system, but if there is it’s from the master cylinder sucking it in, and the cure is to replace the master cylinder.
The pads, rotors, and calipers have nothing to do with this symptom.
There’s a chapter pretty much exactly like this In All Creatures Great And Small. Right down to the dismissal until near tragedy, then the ass chewing “Why did you let it get this bad without telling me?” Also from that book, and something I’ve told my own kids: A good driver doesn’t really need brakes. Just pay attention, plan ahead, keep downshifting. 
And yeah, master cylinder. Or a hole in the brake line and the fluid’s drained out–that’s always good for laughs.
Does your pedal stop moving when you step on it or does it slowly sink to the floor? If it just stops low, it could be the rear adjusters. I’ve had this happen on cars with rear drum brakes. It was because the automatic adjusters didn’t adjust the shoes. I freed up the adjusters and set the clearance on the rears and all was well.
I suspected that the rotors wouldn’t malfunction like this. I’ll be wary if the brake shop says I need a FULL brake job with rotors, pads etc. At least get a 2nd estimate elsewhere.
I looked at the master cylinder carefully when I checked the brake fluid. No sign of any leak. But it still could fail internally.
That’s why it’s so insidious. There is an internal plunger, sort of like inside a syringe. When the seal on that plunger fails it can do so gradually over time, or suddenly and totally. So the onset of a MC failure can look like all variations of severe, but the symptoms are the same: no stable resistance on the brake pedal.
Worse than expected.
Left Front caliper stuck, rear wheel cylinders dripping fluid and rear drum brakes not working.
Front
New pads and rotors. 1 new caliper
Rear
Turned drums and replace rear wheel cylinders
$850 ouch!
Could have been worse. They did save the drums
2000 Ford E150 van with 55,500 miles