Allright heres what happens. I start the truck and drive around for a while. When the engine warms up and stays warm for a while, say 15 minutes after I start it cold, a funny thing happens with my brakes and idle. When I go to stop, it stops just fine but if I stay stopped with it in gear for more than a few seconds, the idle speed comes up to about 1,000 rpm from about 550 rpm all of the sudden. With my engine and torque converter, the truck is very hard to keep stopped which really sucks in traffic because I need to keep shifting it from neutral to drive with my automatic. This seems a little funny to me.

Heres my setup
I have a 77 Ford f150 4x4 with a swapped in 429 and C6. The 429 is mildly built and am not running a radical cam or anything. The brake system is stock vacuum assisted power brakes. I have my vacuum hose from my power assist unit to the port on my Edelbrock 750 Performer carb. At first I thought that it could be a vacuum problem so I changed the line and put it on the manifold port instead, but no real change. Is this still a possibility or is it a carb problem. I am at a loss. The carb seems to be tunned right and all my vacuum lines are either plugged or in use.

Any opinions as to what my problem could be. The truck is kind of dangerous to be driving in traffic like this.

 

When I go to stop, it stops just fine but if I stay stopped with it in gear for more than a few seconds, the idle speed comes up to about 1,000 rpm from about 550 rpm all of the sudden. a little funny to me.

So what happens? Do the brakes let off? Does the engine out pull the Brakes?

 

 

well i am making around 530 ftlbs of torque at around 1400 rpm so, the engine outpulls the brakes.

 

If I'm understanding, you are saying that at a stop light, your fruck starts to Idle up a little higher, all by itself, and when that happens, the Brakes cant hold the Truck.
So you have to put it in Nutral to allow the Brakes to hold the Truck in place.

If that's all correct you have one real severe Problem. Those Brakes are in Bad shape! If the Brakes cant hold the Truck against a strong endine at a Stop, How do you expect the Brakes to stop your Truck from 60 MPH?

You are going to find something during an inspection of your brakes. Burned or cooked Shoes, DRums, Pads, Rotors, and you might even find some frozen wheel cylinders.

As to why the Engine starts to rev up, it sounds mechanical, like a Dashpot on the Carb, or your foot holding down the Brake pedal so hard the side of your foot is on the gas pedal.

I'm curious about all this Torque at such a low Rpm. How do you know you have this much torque? Did you have this tested on a Dyno?

 

Before I replaced the entire brake system below the master cylinder, the front brakes were the only brakes that worked. I just recently redid the brakes and got done with that on saturday. I replaced the front callipers, pads, all brake lines, rear wheel cylinders, shoes, and the proportional valve. When the front brakes worked only, the truck would either sit there doing a burnout (if it was at all damp outside) or simply push itself forward. That is why I realized that the rear brakes were not working. As of now the brakes work perfectly and will stop me just fine but the idle speed still comes up.

 

Did you check for a hole in your brake booster diaphram (spell?)When stepping on your brake you are calling for vacumm at the diapram it can't do it so your idle goes up with a major vacumm leak and you lose your Power on your power brakes?? my 2 cents good luck

 

Right on GP, that was next, but he's still going to replace the Rear drums. They are Cooked! The metal has lost it's tension, and they expand with pressure.
And then he's gonna put F-350 front disc brakes on, and then he's gonna stop racing between lights.

When he goes at the Diaphram Job, he might as well put the whole works from an F-250 or F-350 on, all at once. Master Cylinder, Diaphram, Front Calipers and rear brakes! Find a Donor truck and get your good brakes.

Years ago I knew a kid with a Chrysler 300-G in an old Mercury.
2 Four BBls and all that stuff. When he finally rear ended a Truck and ruined his motor and all that stuff, he admitted it had been having a hard time stopping. He just didn't want to quit racing long enough to fix his brakes. You could spend a lot of money on fast rides at that rate.

 

First off, my brakes themselves work perfectly fine. My truck stops very well for its weight. It comes to a complete stop just fine now that I just redid the brakes, everything. It is after I stop that things get a little unnerving. BTW, I do not race between stop lights. I have never even revved the truck above 3000 rpm and that was while fourwheeling. On the street, the truck rarely goes above 2500 rpm.

GP, your idea sounds about right, and I might just go check that out. That makes sense. I might just do that. I have talked to others about carburator problems, but I have checked to make sure that the linkages on the carb are not sticking or anything like that and they are not. This does seem like a vacuum leak to me, but when I stop from 2000 rpm on the road while driving the power brakes seem fine. Let me now your opinions. Also, the vacuum booster from any of that era F series trucks are directly swappable, correct.