Hello everyone, I am at a loss so figured I would ask you all! I have a 78, with the 300, manual. When I fire my truck up in the morning or anytime on a cold start, it stumbles a little bit when I have my foot on the brakes. Almost like its being choked. As soon as I let my foot off the brakes the idle comes back up to normal. If I start driving not completely warmed up, the pedal is very stiff and drops the idle when I push on the pedal. As soon as she is fairly warmed up all of those issues go away. A couple months ago I had the whole braking system gone through, from a new proportioning valve, to lines, resurfacing, and a master cylinder. I would think all of this would point to a vacuum leak? Or maybe the booster on its way out? But I didn't think those would go away just with the motor warmed up. It doesn't effect my everyday driving, I just have to let her warm up a little longer in the mornings. Any ideas?

 

plug the line to your booster and see if it cures it.

 

That was actually on the to do list! Should narrow it down

 

Your brake booster is failed internally.

 

My mom had a 73 Duster that did the same thing years ago. The brake booster was bad. Changing it fixed the brakes and rough running when cold. I suspect that as the engine compartment warms up, the expansion of materials in the booster ends up sealing the spot where the vacuum is leaking.

 

Yep I think you're correct. This morning on a cold start I put my foot on the brakes and the idle went a little higher, let it idle for about 20 more seconds, pushed the brake pedal and the idle dropped lower like it wanted to quit. Then after a couple minutes of warming up 0 issues and the brakes worked like normal.

 

Yeah that Duster used to always be worse in the winter... taking longer to warm up the engine compartment. In the summertime, the booster would start to work after just a few minutes. But when that booster was cold and leaking vacuum, step on the brakes and that engine would shake like crazy!

A new one will cost you about 75 to 85 dollars for an F100/150. I just bought one for my '78 F350... was 98 dollars.

 

Luckily I am in phoenix so even on the coldest mornings it will still be right under 40. Fairly easy to swap out for someone with limited knowledge? I had the whole braking system gone through a few months ago. I guess hindsight I could have just replaced the booster when everything else was done, but at the time no need to throw another $100 in if I didnt need to.

 

Yeah it's pretty easy to change out. Don't open the master cylinder or disconnect those lines... then you won't have to bleed the system again. just use a piece of line to tie it hanging up from the hood and remove it from the booster. I think there's only 4 nuts holding the booster to the firewall.

 

Just a quick Thanks to OP and responders of this thread ! I've been having the same problem, cleaned components and verified leak b/w Brake Booster and Master Cylinder, sourcing Dual Diaphragm Brake Booster (F-350) and replacing today. This has been driving me nuts!!!

 

Martin... Where did you find a booster for the F350? Usually, you have to send your bad one off to be rebuilt and they send it back to you.

 

@CharlieXLT- Man, why you have to scare me?!?! I was enjoying a nice coffee at the Coffee Shop, minding my own business looking for one, and now I'm worried. This is my daily driver that I just picked up and started restoring, so kinda do it as I roll along, so to speak, I'm gonna be really bummed if I have to send it off! Share some wisdom though, don't worry about my delicate sensibilities. The Factory Manual calls it a "Tandem Diaphragm (No size given, but believe it's 9 1/4"?), so I have been looking at "Dual Diaphragm" (Not sure if this is the catch) Boosters through a Google search for Local Auto Parts Stores, who show options for what they say is a replacement. I have noticed that there is a Boot on the back-side of the ones I have seen online (Caveat- repeatedly being sold parts that are incorrect or for a 351W, not 351M, has been a recurring theme), I've climbed up on top and really can't get a good look at the back side. Regardless, safety is a big concern for me, so I want to do it right, and if possible, improve the Braking performance by getting a quality Booster. I would VERY much appreciate some guidance though, if you have some to give, to avoid the situation of me pulling all this apart and realizing that The LPS has me twiddling my thumbs and scratching my noggin...

 

i have a 78 350 that i've been wanting to replace the booster. So far, all roads I've been down are telling me not available on anyone's shelf... send in the old one and they rebuild it. I do not know what kind of turn-around time they are figuring. Mine doesn't seem to have a vacuum leak... but instead weak springs. The pedal does not come all the way back up to the resting position. I have to give it a little tug with my foot so that the brake lights will turn off.

I toyed with the idea of just putting a 150 booster in there... if it will work. Those are readily available. I am sure the mounting hardware on the front of the booster and the firewall is the same... but not yet sure about the pedal pivot assembly... it's possible the 150 and 350 pedal assemblies could be different. I will be making some comparisons between my trucks (I have a 100, 150, and 350). More research will be needed before I go that route.

 

Of course, this doesn't mean there isn't one out there for a 350 dent... you just might have to do some more digging around than what i've done so far.