Soft brake pedal - no fluid loss

My brake pedal feels as though there is air in the line because the pedal goes soft and I need to pump it to brake the truck. However, I see no loss of fluid. Everything looks fine. What could be causing this? Bad brake master cylinder? 1987 pickup - 22R - Carb - 69,000 miles. Any help would be greatly appreciated.

Check to make sure the rear brakes are adjusted properly.
There could be an internal leak in the master cylinder. If this is the case then you will have to replace the master cylinder.

Signs of a MC internal leak are that pedal sinks towards the floor after initial pressure is applied. It fells normal/hard at first, then sinks under pressure.

If it is spongy at first then gets hard/normal after you pump it, that's typical of air in the line, or rear shoes need adjusting as stated above.

Thank you KimMG. I pulled all four wheels and checked again for leakage. There was none. I went ahead and bled all four wheels and pushed through new fluid. But, that didn't help. I plan to replace the master cylinder next. Hopefully that will fix the problem. If not, then its down to the wheel cylinders.

Were the rear shoes adjusted properly? I ask because you did not mention checking the adjustment.
How old are the flex lines? They deteriorate internally with age and can slightly balloon when you apply the brakes. Blockages can also occur preventing you from properly bleeding the brakes.
If the wheel cylinders are not siezed and are not leaking, they are not your problem.

Thanks again KimMG,
The flex lines are as old as the truck 19+ years. I have not jeu checked the adjustment on the rear brakes.