Brakes LOSE power when engine is on! Booster problems?

[fordhombre]

Okay, here's the skinny. I have a new (to me) brake booster. I have not ran the pickup with the new booster attached until today, but previously, the brake pedal had good pressure going down. Then I cranked the pickup, and the brake pedal was flimsy!

Is this a faulty booster? Is it possible I have connected the vaccuum line into the wrong end of the carburator? It's a 67 brake booster on an Edelbrock 4 barrel carb. The vaccuum line is connected to the back side of the carb. There are two possible line connections in the back of the carburator, and it's currently connected to the bottom nozzle. The top nozzle right now has a little nipple cap on it. Is this to prevent unnecessary vaccuum? Could this be it? Any help would be MUCH appreciated! Thanks!
-Stephen

[FORDification]

It's possible that the place you've hooked your booster is a ported-vacuum port...not what you want. Ideally you should connect the vacuum line to a nipple screwed directly into a runner on the intake manifold. The port shown in the photo below is where there is normally some form of vacuum connector, and is normally where the booster and the automatic transmission vacuum modulator gets their vacuum from.

[fordhombre]

Is it okay to feed the transmission line and the brake line on the same nipple? With like a splitter or something? Or will that weaken both the Tranny and booster. It is an automatic transmission on a 351w.

[FORDification]

Actually, they're usually hooked together off the same port on a T-block, with a small nipple going to the tranny and a larger one for the booster.

[jor]

My Edelbrock 750 has one port on the back side for power brakes. In my experience (I also have a 67) the booster needs a dedicated vacuum source. I tried to "T" mine with one side going to the booster and the other to the PCV valve but that resulted in a really inefficient booster. When I went back to a dedicated vacuum source it worked fine. Like Keith suggested, if you have a source on your intake, use it.
jor

[67F250CS]

Agree with that. I have the Edlebrok carb also. I flipped it over and that bottom port is free and open to the intake but its too small for my liking. If you have any sort of non stock mild to wild cam it will effect the vacum at idle. I am going to run my 68 style booster directly to the manifold at the back. I found the booster and M/C at a local parts store and mocked it up to my 67 pedal assembly it put the pedal 1/2 inch higher. The rod out of the booster has that special cut out to activate the brake light switch.

[fordhombre]

Okay, I cranked the pickup again this morning and listened carefully. When the truck was at idle, I pumped the flimsy brake pedal to find out that it actually sped up the idle! Maybe it's pumping air into the carb?

For you other guys out there with Edelbrock carbs, do you know what the top nipple on the back of the carb is for? Could this be a possible replacement?

Thanks for all the help!

[67F250CS]

Ive got the book that came with the Edlbrock. I'll dig it oout and post some info and maybe scan the pages and post. Im off to work right now now but sometime today I should be able to post it.

[FORDification]

If the idle speed picked up when you apply the brake, then it's likely you have a bad diaphram in the booster, allowing a vacuum leak.

[67F250CS]

Hers the photos I promises from earlier. It looks like the small one is ported and the larger one is not and is for the PCV hose.

[67F250CS]

Darn I posted the wrong pic and cant replace it or add asecond one to it for some reason. Maybe a size limit thing. I'll try again later to get the second one. Its the bottom half.

[67F250CS]

Trying the second page again.