Hi to everyone and I hope everybody is fine.
These can include an overheated braking system, using the wrong brake fluid, damaged or broken parts (calipers, brake pads, pistons, rotors, or others), a defective ABS component, broken parking brake, and more.

Hi mate,
Some of the most common underlying issues include faulty brake pads, bad calipers on the disc brakes or faulty cylinders on the drum brakes. ... Caliper piston problems, master cylinder issues and a variety of other problems can cause your brakes to lock when driving normally.

Yep. They are different. I also learned the hard way. 45 years ago I rebuilt the clutch & brake MCs in my '59 MGA. It had a single cast iron unit containing both the clutch and brake cylinders. They each had an identical looking little valve—and the valves were interchangeable no less! After the rebuild and during a 10 minute test drive, the clutch was slipping and the brakes were overheating. What the heck? I called my local MG guy and he told me about the valves; one allows pressure to build as the peddle is pumped (the brake valve), the other does not (the clutch valve). Who knew?

As for 'Rosebud,' this time around it was a simple MC failure. A new unit put things right. 

Glad you got it solved. When you mentioned that the front brakes were locked up, something else came to mind. I'm not sure if anyone else is that aware of this issue, but the metal plate that sits under the Master Cylinder is a required item. Several years back (at MME) a friend of mine had just restored his Mini van. Everything was brand new and looked great! When he took off in it, after about 2 stops the brakes were locking up and the only way he could get back was to pop open the bleeders. While there (and parts were available) he changed out the MC and hoses. Still was locking up. When we looked under the bonnet Hotelcalif (David Kimble) noticed that plate was missing. When questioned about it, it was left out because it didn't look good and seemed unnecessary to the build. Here's the skinny in case it comes up to anyone in the future.

First off, the Brake MC is different internally than the Clutch MC. They may look alike but the are not interchangeable. That is the first question you need to ask. Secondly, the difference is that there is a hole drilled in the cylinder that allows the fluid to return to the can when you release the brake pedal. Because the Clutch MC doesn't have that hole, the pressure to the brakes will not release. Also, if the plate under the MC is not there, the MC sits just low enough that the piston in there will not drop low enough to uncover this hole. Seems like an odd thing, but it is true. Since he didn't have the plate with him, we solved the problem with a couple of flat washers (maybe 3 on each stud) to raise it up to where it needed to be. I'm sure he fixed it properly when he got home.

Here's something else which I will add to "stupid things I have done"...probably 20 years ago after changing pads and rotors on my Dodge Minivan, I grabbed a bottle of what I thought was brake fluid to top everything off. After about 30 minutes of driving my brakes locked up. Again, they wouldn't release until I popped open the bleeders. When I finally got home I double checked the bottle I used, and it was actually Power Steering fluid. Damn similar bottles! No damage done, but I did have to evacuate ALL of it out of the lines and replace, which took most of the rest of the day. Note to self: pay attention to what you are pouring into anything involving your car.

Problem solved. I installed a new master cylinder and the front brakes are working fine. My old MC was less than 2 years old when it failed. I checked the warranty and learned that "due to the stresses of competitive driving and variables in DYI installations," Welwood offers no warranty whatsoever. Perhaps other manufacturers take a similar position. I guess I never noticed.

The entire braking system has been in place for 20k miles, so nothing new. This afternoon I ran a new brake line from the front MC to the front brakes, bypassing the booster, and the peddle still goes to the floor. I bleed the front brakes and get good fluid flow from each. Unless I'm overlooking something, all signs point to the front MC as being the culprit. I ordered a new MC that should arrive Sunday. Will advise…

Here's a little more info: Although the front brakes have failed, I still have rear brakes, suggesting the front MC or possibly the booster has failed. The fluid in the MC remains full, so I'm hoping that means my booster hasn't failed and filled itself full of fluid.

Years ago I had to replace the flexible brake lines at the wheel in my 72 Porsche.  The deteriorated and would plug up the line causing the the brake to freeze.

If you used an R clip on the clevis pin at the pedal to master cylinder connection, you may have pushd out the R Clip and lost your clevis from the master'pedal connection.

BTDT

I no longer use R clips there.

Is anything in the brake system NEW? How old are the front flex lines to the calipers?

I once had a new master that was not built right, the push rod was too long. After driving it for about 10 miles the front brakes were locked. The only way to move was the release some of the brakes fluid at each wheel. The push rod being too long would not let the master go fully to the rest position and unblock the drain back hole to allow the pressure to release.

The flex lines get old and swell up inside and restrict/block fluid flow. The master is strong enough to push it in but the return is blocked by the swollen hose.

The same as Dan. I know nothing about boosters.

I have no experience with brake boosters.
Not a definite answer - just thoughts to get the investigation going. 

Since the brakes locked and then the pedal went to the floor (and you have a split system), I'd suspect a major failure in the mast er cylinder. Hypothetically, you applied the brakes and they did not release, suggesting that something in the master cylinder locked in position, and you eventually rattled it free. When you next used the brakes the pedal went to the floor suggesting that the master cylinder is incapable applying any pressure, With the split system you describe, and it wasn't the MC but somewhere else, you should have had some braking application to the rear.

Check the brake fluid reservoir. If it is full and the MC does not move any of it, that would be a srong indication of it not working. If it is very low or empty, the MC may have pumped all the fluid into the booster and it may be the culprit.

My front brakes locked up today. I felt the pedal getting hard as I was sitting at a red light. By the time the light turned green, the pedal was completely hard and the brakes were locked. I turned the engine off and repeatedly pumped brakes. The pedal finally broke free and went to the floor. I now have no front brakes. I’ll be driving home using emergency brake only. Anyone have any thoughts on where I should start looking? Here’s my set up:

Two separate brake circuits; one for the front discs, one for the rear drums. Vacuum booster on the front only. The fluid looks fine, no visible fluid under the hood or on the ground.