1996 Ford E-250 Lost Break Pressure

Okay, here is the issue I'm having. My 1996 E-250 hasn't had any brake issues since I've owned it for the past couple of years, until a few days ago. I was driving with nice solid brake pressure until all of a sudden I lost nearly all pressure and had to slam the brakes all the way to the floor to get the van stopped. I checked the fluid level (which was just a little low) topped it off, and then started driving again without any issues. Then, yesterday the same thing happened, lost all pressure and this time i heard a knock under the front of the vehicle like something fell out or hit hard. The brakes firmed up and I drove it carefully home.

When I lost pressure I didn't notice the van stalling or losing power in any way. Also, I can get a firm pedal when the van is off after pumping the brakes and the pedal doesn't seem to creep down if I hold down while off. Also, there is no fluid loss at all. Basically, now I can get the brakes to engage if i press down almost all the way to the floor.

I was wondering what the most likely issue is that is causing this problem--master cylinder, caliper, vacuum leak? I can work on brakes fine, I'm just really rusty on the diagnosing part.

I would start off by bleeding the system at all 4 wheels. Start at the farthest wheel from the Master cylinder working towards thel closest wheel. While you are doing this you can look at all the hardware to see if there is any problem. The power assist doesn't work with the engine off so you will have a hard pedal without the engine running.