I HAVE A 77 BRONCO THE OTHER DAY WAS DRIVING IT AND HIT THE BRAKES AND IT WENT TO THE FLOOR. GOT IT HOME TO FIND OUT THA THE REAR LINE ALONE THE FRAME RUSTED THROUGH. SO I PUT A NEW LINE ON TODAY ALL NEW FROM PROP. VALVE TO REAR HOSE. I BLEAD THE BRAKES BY PUTING A HOSE ON THE BLEEDER VALVE INTO A HALF FULL BOTTLE OF BRAKE FLUID. TOOK IT FOR A DRIVE AND NOT ALL THE TIME BUT SOMETIMES WHEN YOU GET TO A STOP SIGN AND HOLD THE BRAKES IT WILL GRADUALLY GO TO THE FLOOR AND LOSE BRAKE PRESSURE. CANT FIND ANY LEAKS ANY WHERE NOW.
WOULD IT MAYBE STILL HAVE AIR IN THE LINES? OR IS MY MASTER CYLINDER TAKING A POOP?

sorry bout the caps didnt realize it was on til i typed all that and wasnt in the mood to retype it all

 

still some air in the line is the 1st, most likely answer - but where?

1) more holes elsewhere (other breaks/lines) or rubber line 'flex'?
2) still @ the point U were bleeding originally (not bleed enuff)?
3) wheel cyl (in back) or calipers (in frnt),
4) @ MC?

solution? not sure. Continue to bleed, visually inspect the whole system for (more) leaks. I'd suggest a test after each component is blead seperatly (or line is replaced). If still soft on the pedel bench bleed the MC, reinstall, test.
A clean cloth or newspapers under can catch leaks as a help in spotting them (ever house train a pup? we can do it with our 'animals' too) for when U walk away from the vehicle.

All is easier if U have a friend pump while U look/bleed.
Post back with your results if you'd like more (in my case) lame ideas
Good Luck!

 

Most likely its the MC... but like GFW says go over the entire line system first to see if you see any we areas.... which could be slow leaks that would cause what you describe to happen under pressure.