My truck is a 1986 f250 4x4 and when I push the brake pedal it goes down a couple inches then gets stuck so I replaced the brake booster and it still does it, what could cause this?

 

I dont know if the booster bolts to the firewall and the rod right to the pedal or a bracket and linkage then to the pedal?
Either way I would disconnect the booster from the pedal or linkage so the pedal / linkage is not connected to the booster.

If the booster is to the pedal and the rod removed move the pedal it should swing free on the cross shaft up top.
If not then that shaft needs to come out and checked for rust & dirt but the pedal needs to swing freely.

Now if you have linkage setup you can do the same, see if the pedal swings freely.
If not you will need to take the linkage part out of the picture from the pedal and test both to see if they both move freely.
If either or both dont move freely they need to be fixed before being put back in to service.

Now if all that dose move freely then I would have to say that booster could be bad. Yea like new parts cant be bad
Or a long shot the master and the piston is getting stuck in the bore.

I do have a project car the booster junk from brake fluid inside had hard to push but before I found the fluid that setup uses linkage between booster & firewall and I found the linkage was rested and would not move. So dont over look linkage.
Dave ----

 

it’s kinda odd when the truck isn’t running the pedal returns but when the truck is running and has vacuum it doesn’t come back

 

Seems your new booster is bad and leaking. Does the motor run rough? I'm pretty sure there is a return spring on the pedal assembly so the pedal returns when the vacuum which is on the wrong side of the piston is relieved.

Edit: there is a return spring in the master cylinder.

 

I remember back when I had to pull the brake pedal back up with my foot, a new booster fixed that issue, I think you got a bad part, it happens a lot these days.

The trusted mechanic, that replaced it, said he had to go through a couple to get a good one. That was about 12 years ago.

 

no the truck runs great fires right up and idles smooth

 

That's a good sign. Take a look at this page for booster vacuum diagnosis:

https://www.garysgaragemahal.com/brakes1.html

 

I might just pull my booster off and return it I’m just not sure why the pedal returns when not running but not when running and I do hear vacuum when I push down on my new booster

 

The vacuum should bleed down when you release the pedal. For some reason it's holding the pedal down.
Follow the diagnosis procedure on Gary's page to determine where the problem lies.

 

yeah, I’ve never had this much trouble with a booster

 

Is there a way I could add a return spring under the dash?

 

You would have to look as I dont remember but something is not right if it dose not come back up on its own.
I would also say the booster is at fault with the testing you did
Dave ----

 

The old booster and the new booster give the same result? I think you need to diagnose further before getting another booster. When did the problem arise?

 

the old booster didn’t come back at all the new booster comes back when truck is not running but sticks when it is running

 

Get another booster. Be sure not to pull the rod out of the booster that goes to the master cylinder.