strange brake problem

[dr.deere]

hi everybody. i have a 94 f-250 4x4. with cruise and a 5 speed manual. the brakes have felt not right since i bought this pickup a couple months ago. they feel soft and if i push the pedal hard it fades to the floor. so i checked the basics fluid level, leaks brake adjustment on rears. everything looked good so i put a new mastercylinder on it today. bled brakes at all 4 corners starting at right rear and working towards left front. no air from any corners pedal feels great. start pickup and back to same problem. so my question is do i have a bad brake booster. its not what i would expect from a bad booster but i'm at a loss now.

[garyeco]

You might try checking the vacuum pump mounted on the driver's side front of the motor.These have a habit of slowly going bad over the years.If you aren't noticing any fluid leaks,the pump might be your problem.Good luck!