So I was driving a couple weeks ago and felt the brake pedal clunk under my foot and the pedal dropped down a lot closer to the floor. Ever since then, it feels like the brakes have air in the lines and I don't really have any brakes until the pedal is an inch or two off the floor. I've gone over the front and rear brakes and the pads and shoes are in good shape and there is no fluid leaking anywhere. Even adjusted the rear brakes until they felt like they were dragging to see if maybe that would bring the pedal back. I also tried bleeding and flushing the brake lines but nothing has really made a difference. The brakes are still pretty good and they don't pull under heavy braking but it's a little nerving to be nearly hitting the floor every time I step on the brake pedal.

Any ideas? I don't want to just blindly change out the master cylinder or booster. This car has 225k miles on it.

 

Have you checked caliper guide pins? Front wheel bearings? Ballooning brake hoses?
Does pedal sink to the floor under light pressure? If so can be master cylinder. You can even try to isolate master cylinder from the lines by plugging the ports with plugs (M10X1mm)
Edit: since you've done axle work,.check the front wheel bearings again! If everything check out and master cylinder holds pressure with ports blocked off (you can do one by one first, then both), the least likely problem (because no copy and paste experts ever reporting such!) will be leaking solenoid valves in the ABS unit allowing fluid to go into spring loaded reservoirs) Something like this happened to this Ford

 

Yes, it's very light pressure. I'll look closer at some of those things but if I have pull brake lines off the master cylinder, it's going to get changed.

 

Yeah my issue is not like that video....thankfully... I'm sure that piece would be tough to find and/or expensive. That video looks like he has a steady pressure bleed in the system where as mine has virtually no pressure until the bottom of the pedal stroke.

 

Probarbly one of the seal in the master cylinder is leaking or blown. Best to get it checked out ASAP. Brakes are no longer as effective.

 

When I took few cylinders apart on the wrecking yard, I've noticed wear marks on the steel pistons (The cylinder is aluminum but pistons are steel) so the bore of the cylinder is no longer round and this makes for cup harder to seal.Again the failing gives you a pedal the gradually looses it's firmness when applying steady pressure (keeping the car at from creeping forward at stop light). Often the "fall of the pedal" is accompanied by groan sound of releasing brake pads.