newbie here...
i have come across a Dakota...it had a few bad lines so i replaced them
now i did have brake pressure at first...now when the truck is running it goes to the floor and no pressure builds...shut it off and the petal gets hard to push

i took the caliper off and it doesn't move...when pressing petal...

 

If your lines were bad, the m/c is prolly shot as well. You might try to re-bleed all the lines also. Can't hurt

 

Ya could be your mc. Or a leak making you loose pressure.

 

When you say you had pressure at first, do you mean to say that you had pressure before you worked on the brakes, or after you replaced the lines you had pressure but, when you started the truck you lost the pressure you had. If the former is the case then I would have to say that the system was not properly bled. Especially if when you press the pedal you get no movement from the caliper. Now if you didn't have pressure to begin with and that led you to discover that you had bad brake lines, then what my forum mates said about the master cylinder. I would check the power brake booster as well. Is there a puddle of fluid under the truck when you press the pedal to the floor?

 

I have a similar problem. When the trucks not running, it has plenty of pressure, but when I start the truck, the pedal goes nearly all the way to the floor before the brakes start to catch.
I replaced the master cylinder and power booster two months ago and let it sit until last week. Today I took it to Brake Masters for a brake flush, but it didn't make a difference except for it made my wallet lighter by $45.00.
When I first started it last week, it wouldn't idle because the pb check valve was leaking around the grommet; but after a while of revving it, the booster built up pressure, sealing the check valve.
The booster and master cylinder are Cardone remanufactured from Checker/Schucks/Oreilly Auto Parts; if anyone knows how to fix this; you will be my best friend forever.

 

If the truck has abs there may be air in the abs system. Not sure of the procedure for bleeding that out.

 

I let it gravity bleed for about an hour, and the pedal was still soft. I removed the master cylinder and found that the power booster push rod was too short, so I adjusted it and alls well now.