1986 5.0 brake problems...replaced brake booster twice and master cylinder once....now there is no power to the brakes...the brakes are stiff while the care is off....but when the car is on the brake pedal goes all the way down...blead all lines and blead master cylinder...have no clue as to what else to do...please help...this is my daily driver

Most likely a failed master cylinder or vaccume leak to the booster. Here is how to check for a bad brake booster first with the vehicle off depress the brake pedal usually once or twice is sufficient but however long it takes to make the pedal as firm as possible now with your foot on the brake pedal pushing down start the car if the pedal moves your boosters not at fault it's your master cylinder or another problem if it doesn't move its most likely a booster problem

This is a standard Vacuum operated brake booster.. If you are having fluid leaking at the rod that attaches the brake pedal to the booster it is a bad Master Cylinder. The Brake fluid will eventually destroy the diaphram in the booster if it hasn't already. . Replace or rebuild the Master Cylinder and wash the brake fluid out of the booster. When the booster goes bad the brake pedal will become very hard to push.. because you will have lost "boost" This is not really a hard job. If fluid id leaking somewhere else.it should be very easy to spot.. Brake fluid looks like oil leaking..

Did you replace the master cylinder too ?
What was the problem with the old booster ?

master cylinder is in the engine compartment on the firewall in front of the driver seat. Stiff brakes do not indicate a bad master cylinder, but could indicate the booster. Listen for vacuum leak at the booster with engine running. The booster is a big donut shaped thing behind the master cylinder.

you should replace the booster and master cylinder. With brake fluid there ,the diaphram in the booster will be damaged .

leaky rear break cylinder or master cylinder

Did you bench bleed the master before you installed it???? Engine off, have someone push the brake petal down and hold it down while you crack the brake line loose at the master,then tighten it and let up on the petal.Do this several times for each of the 2 lines at the master, only cracking it when the petal is being held all the way down.Repeat...Do them separately.Have some rags below the master to mop up the mess and keep the master from running out of fluid. Then do the gravity bleed one more time on each wheel cyl. and watch for a bubble of 2.
I hope this helps you.
Yes the booster sounds fine.

A bad brake booster will result in a hard brake pedal, stiff and more effort required to stop the truck. Here are my questions to you, first, you said you replaced the master cylinder, did you bench bleed it before you installed it? If not, there IS still air in it, regardless of how much you bleed the brakes at the wheels. Second, when you bled the brakes, after changing the calipers and master cyl., did you also bleed the proportioning valve? Sometimes air can get trapped in the valve, and its almost impossible to get it out bleeding just the wheels. And lastly, when the brake light comes on, does the abs light also come on with it? Air can also get trapped in the abs control module too.

check master cylinder around vacuum booster for fluid leak .also if pedal is stiff it could be vacuum to booster or booster itself..master cylynder connects to split block of brake lines,there should be a sensor on that block..see if connected good,, chris

get ready for some work.you need to remove the master cylinder nuts that hold master cylinder to brake booster.once you remove nuts you have 2 choices remove master cylinder to remove booster. to keep from bending brake lines.or you can leave master cylinder in pull it toward you easily careful dont bend lines .disconnect vacuum line at booster .go inside vechicle to floor on driver side loosen four nuts on floor to booster .disconnect power brake push rod from brake pedal .dont force pushrod to the sides when disconnecting it.remove four booster nuts then remove booster.when in stalling the new booster loosely install four nuts then connect the push rod to the brake pedal .install new clip tighten the booster unit .if you decide to remove master cylinder use inline wrenches on master cylinder to remove lines.also plug the master cylinder brake line holes to keep from losing all brake fluid.