Hydraulic trailer brake controller causing brake issue?

[Dodonne2]

Hey y'all. I've been having a heck of a time trying to figure out my brakes on a 1984 6.9 IDI f250 2 wheel drive. I've replaced brake booster three times, thinking I was getting faulty boosters. I've replaced the master cylinder and bled the brakes two times now. The symptoms I'm getting are all over the map, the pedal used to not return and I would have to reach down and pull it back up. I put a new master cylinder in it and that solved that problem, but now I have no vacuum except for the first quarter of the brake pedal the last three quarters has no vacuum whatsoever and I have to push really hard to stop the truck. I've done it vacuum test and I'm getting 20 HG consistently with the truck running on the line coming into the brake booster from the vacuum pump This truck has the original trailer brake controller that is hydraulically driven connected to the master cylinder. My question is could this be the issue with my brakes? when I push on the brakes I'm noticing that the trailer brake controller knob is moving back and forth. how do I bypass the system in remove it? Do I just plug up the port at the master cylinder And re bleed the system?

Thanks¡

 

[chillman88]

The way you describe the issue it doesn't sound to me like that's the problem, but it's easy to bypass. They're usually installed using a T connector, just unscrew the factory brake line from the T connector and then remove the T from the master cylinder, then reinstall the factory brake line and bleed the brakes again.

 

[IDIBRONCO]

The way you describe the issue it doesn't sound to me like that's the problem

I have to agree with this. I may be wrong, but it sounds like it may be a brake booster issue.

 

[Fixnstuff]

If I remember correctly the lower limit for the vacuum pump is 19 in. Hg.
A good one should pull 24 in. Hg. (That's what mine tests at). Can anyone confirm that lower limit?

Due to slightly lower atmospheric pressure in the mile high city of Denver, vacuum readings would be slightly less. I did those calculations once and the difference was very small and I don't remember the actual difference.

I assume that you also have a C-6 automatic so there are vacuum lines associated with that and a vacuum line to the blend door motor for the HVAC system. You might have a vacuum leak. I would check the vacuum hoses under the clamps at all of the connections to see if it's dried out and cracked under the clamps to make sure you are not losing some vacuum at such a location. Usually I just cut off the bad end and reattach it until I get around to buying new vacuum lines to replace them.

Otherwise, I think that 20 inches of Hg for vacuum is getting down to a level where the vacuum pump performance might be a concern.