Vacuum boosters don't make soft pedals. Quick test: engine off, pump pedal several times to deplete vacuum reserve. Hold foot on pedal, about 5# force, and start engine. Pedal should drop, indicating booster works. Pretty simple.
Bad master cylinders don't make soft pedals. Falling pedals, pedals on the floor, frozen pedals, etc. could be master cylinder defects.
Bad brake hoses, that expand under pressure, can make a soft pedal.
Really cheap, soft rotors and/or drums can make for soft pedals. Over-sized drums can too.
Cheap, crappy pads can make for soft, spongy pedals and/or extended stopping distances. Some pads are so cheap - not inexpensive - (and believe me, I've seem some name brands straight out of the box do this) that you can actually see the friction material compress while someone stands on the pedal.
Loose caliper mounts and hardware can make for low/soft pedals as can loose/worn wheel bearings.
Physics, anyone? What's the difference between gases and fluids? That's right, fluids don't compress, gases do. Air in the brake fluid will compress and give a soft pedal. Caveat: heat a fluid past its boiling point and.....? Right again, it becomes a gas.
Brake fluid notes: DOT3 or DOT4 is more than adequate for anyone here on the highway. DOT5 silicone fluid is not compatible with DOT3 or 4, requires complete replacement of all rubber items in the hydraulic system, including ABS components and thorough flushing of the remaining steel components. DOT5 is NOT recommended for street use for one extremely large reason: It does NOT absorb moisture! Think about it... moisture is in the air, there's no getting around it. DOT 3 & 4 brake fluid is like any other petroleum based fluid, it's hygroscopic, meaning that it attracts and absorbs moisture. DOT 3 & 4 hold this moisture and disperse it throughout the entire system. Here's where it gets good: not all of the brake fluid in the system is subjected to the extreme high heat found in the calipers and wheel cylinders so the highly dispersed moisture has little effect on brake pressure because of steaming and compression. Now, in a DOT 5 system, any collected moisture eventually makes it way to the lowest points in the system, being that it is heavier and can't be absorbed and dispersed. So, what's the lowest/furthest points? The wheel cylinders and calipers, of course! Now you have a high concentration of moisture exactly where it will be exposed to the highest heat in the system and create the largest problem. DUH!
What about race cars, you say? Well, they only get driven about 500 miles before they get new fluid and generally are chucked at the end of the season, so moisture in the fluid isn't an issue.