bleeding master cylinder and other issues?

[Captain Ledd]

I got an '84 ranger and I've done some upgrades to the brakes.

D35 swap from a '95 Ranger, the rear brakes are calipers for a full size Chevy.

this is a strictly offroad rig, virtually no street driving,

When I press on the brake it firms up really quick and then will slowly sink to the floor at a fairly even pressure and speed.

Is there air in the lines? I bled all of the lines already and started from the furthest away. All of the brake lines were replaced from the master out. I just saw a post about there being a proper way to bleed a master cylinder but I couldn't find anything on it? the whole reason I redid all of the lines is because a hard line popped when my brother mistook it for the brake pedal (automatic guy, it also never ran with those lines while I had it fyi, got it out of a junkyard). so it had been completely drained.

I'm thinking there's air somewhere but I don't know where.. And the rear disks will be going back to drums more than likely when my 8.8 is done.

[metalmacguyver]

if you do this repeatedly, is the fluid level in the reservoir going down? you may have a tiny leak some where. perhaps a fitting that needs to be tightened some more or a cockeyed flare on the end of one of the lines.

also (i am speculating here) it could be a leak inside the master cylinder.

it doesnt sound like air in the lines because they are soft at first and then the pedal gets firm after a pump or two and it stays that way until the brakes are released.

[Captain Ledd]

nope, no leaks. It's been sitting with "finished" brakes for a good month now (need time to go home and fix other stuff now), no new fluid drips. It's not decreasing from the master cylinder either (though I haven't checked as it's been sitting, only while bleeding and trying to figure out why it still went to the floor)

[michowski]

I would fill the reservoir and rebleed them all again. Chance there is still air inside the lines. Keep a good eye on the level of the reservoir and notice were its at after you bleed them and when your driving and trying to test the brakes.

[Chic N Stew]

this happened to my truck about a year ago, i would bleed the brakes and about a week later, they were spongy again, so i replaced the master cylinder, it was leaking but there were no visible signs of it, so i would maybe think about replacing the master cylinder if nothing else works.