You can jack the car up and have someone press the brake pedal. See how much resistance on each wheel and if it is a lot different. It should be able to lock up each wheel. If you do not have any resistance, that wheel cylinder is bad. If they are all old, might be worth it to just replace them all. Also, how does the fluid look? Is it gold color, brown, black, or? Possibly if it is real dirty and old, could be clogged up. I think you said you changed all your soft lines? If not, possibly one is bulging when you apply brakes and are loosing pressure to that cylinder.