ENGINE RPM DROPS & ENGINE DIES AT IDLE & BRAKE PROBLEMS

Guys I have a 78 F150 that I just built a 460 for and converted to F350 power brake booster, master cylinder, rear wheel cylinders and 77 Thunderbird front brake calipers. I need some help figuring out two issues I’m having. Everything on this engine is brand new, so it can’t be from old or worn out parts.

1. The 1st issue I’m having is with it stalling…When I’m driving and push in the clutch to come to a stop, or even just rev the engine while setting at a stop and let it come down to an idle, the RPM’s will go all the way down to 200-300 rpm and sputter and die. Sometimes it will recover and settle down and idle at 800 rpm, but most of the time it will die. When it does recover, the rpm surges up and bounces around a couple times before the idle settles down and becomes steady at 800 rpm. I started noticing it doing this when I installed the new power brake booster and master cylinder. So I was assuming that maybe the power brake booster was defective and had a vacuum leak in the diaphragm or maybe the cam didn’t have enough vacuum to operate the power brake booster.

I did a vacuum test on the engine and the results were 15” Hg at 800 rpm and 20” Hg at 2000 rpm. I read online (http://www.classicperform.com/techbook/braketroubleshoot.html) that engine vacuum to run a power brake booster should be “a minimum of 18” of vacuum”. It doesn’t state at what RPM that 18” needs to be, but I assume it is at idle. Given that assumption, I’m lacking 3” of vacuum to meet the minimum.

So I pulled the hose from the power brake booster off the vacuum port on the intake manifold and plugged the vacuum port on the manifold and fired up the engine. Then I revved the engine and let it come down to an idle and it did the same thing; idle went down to 200-300 rpm and sputtered and died. A couple times it recovered, but most the time it died. So it can’t be the power brake booster because I unhooked it from the vacuum port and the engine still died.

I’m totally at a loss here and can’t figure out what’s causing it. It wasn’t doing this at all until 800 miles into the engine build and until after I installed the new power brake booster. What could be causing this? Is it a vacuum leak somewhere? Is it a vacuum advance issue with the distributor? What could it be?

2. By doing this upgrade to F350 brake components, it’s supposed to give me 40% better braking (bigger pistons in the calipers, bigger wheel cylinders and a dual diaphragm power brake booster). However; the braking is absolutely no better than it was with the crappy stock brakes! The pedal feels soft also. I did bench bleed the master cylinder for about 15 minutes; however, I didn’t get hardly any air to come out of the large reservoir (front brakes) and 99% of the air was coming out of the smaller reservoir (rear brakes). I then bled all the brakes starting from the passenger rear wheel, then the driver’s rear wheel, then the passenger front and then the driver’s front. I used brand new Dot 4 brake fluid and bled the brakes until the new clear fluid came out with no air bubbles. The only thing I can figure is maybe there’s still some air in the master cylinder???? What could be the cause? I figured it was the power brake booster having an issue, but I was able to eliminate that as a possibility when I disconnected the vacuum hose and the engine still died. Any ideas why my brakes don’t stop any better than the crappy stock brakes did when it’s supposed to stop a lot better?

If anyone is interested in the extremely detailed article I followed to do this brake conversion, refer to this website: http://www.ford-trucks.com/article/idx/0/092/article/complete_brake_upgrade_for_1978_bronco

On a power brake system,if the booster isn't doing it's job ,you will have braking issues for sure.Sounds like that is one of your issues for sure. On the idle issues,some of that could be the problem also,but have you tried another carb just to see how it acts? And where are you pulling your vac from for your dist.?

If the brakes are free of air and the booster is bad it will have a rock hard peddle and be very hard to stop.

If the peddle is soft it more than likely air still in the system. If you are doing this by yourself and don't have a bleeder sytem to pull fluid through, you can just gravity bleed the system buy taking the top off the master cyl. and break one bleeder at a time starting with the one farthest away from the master cyl. Then just let it flow out until pure fluid is running out, then do the rest with next fartherest on down the line till the last one.

My first thought was the carb too. Screw in the idle screws, one at a time, and see if the rpm's drop. Back them out until u reach max vacuum . Just an idea that wont cost anything.. If screwing them in or out doesn't effect the idle or vacuum , u have a bad carb. Good luck.

Update!

I increased the idle up to 900 rpm and it seems to be doing better. It doesn't die anymore when I let off the throttle, but it will shut off without trying to keep running. I guess 900 rpm is the sweet spot for my engine. However, I'm converting it to FAST EZ EFI, so the brand new Holley Street Avenger 770 carb is coming off. I'm willing to sell it if you know anyone who wants a Holley carb. It even comes with the steel fuel lines with a fuel pressure gauge.

However, my brakes still suck after doing the F350 upgrade. They aren't any better than they were stock. I'm going to take the truck to Les Schwab and have them pressure bleed the brakes just to make sure I have all the air out and see how that goes.

Glad you found the problem.

If I may offer a suggestion, it sounds as if the carburetor may be a bit lean at idle. When you increase the primary throttle blade opening, you also may be getting a bit of fuel from the "transfer slots", therefore increasing the amount of fuel at idle. Also if it attempts to "diesel" or continue to run after the ignition is turned off, this may be a combination of throttle blades open too far and an inefficient combustion chamber. I would check the air/idle adjustment screws. If no change in idle quality occurs during adjustment, there is a clogged air-bleed. Clean the air-bleeds and try again. One other thing to try, with idle speed very near a speed which stall occurs, simply work the accelerator fuel pump by hand, not the accelerator which operates the throttle blades. If the idle speed increases, it may be lean at idle. Also an increase of primary main jet will assist in idle quality in an extreme lean condition.

As far as your power brake issues, it may be a side affect of poor idle quality of your engine.

Dave.

I just had my brake system pressure bled by a shop and the peddle isn't as soft as it used to be, which is good! But they got only a "little bit" of air out of the system. However, after driving it I have to say that I'm still not impressed with the brakes whatsoever. Everyone tells me that doing this F350 brake conversion will give me A LOT better brakes, by about 40%. My brakes aren't any better than my factory brakes from what I can tell so far.

I have installed a new F350 dual diaphram power brake booster, F350 master cylinder, 1977 Thunderbird calipers (which has a bigger piston than my stock caliper), and F350 rear wheel cylinders. With this setup, the article I followed to do this conversion states that by using the dual diaphram power brake booster and the bigger pistons in the calipers will give me a lot more clamping force; therefore, making my braking a lot better. This has not been my experience so far. I did order some cross drilled and vented rotors and new ceramic brake pads that I have yet to install. I'm hoping this will help. But in all reality, if the system I just installed is supposed to give me 40% more braking (because of the increased clamping force), then I shouldn't need to install new rotors and pads to get the brakes to work like everyone is claiming....

Can someone tell me what I'm missing and why my brakes aren't any better than factory? I've done everything that the article told me to do and so far I'm disappointed.

Your not alone on the brake problem. I too am having the same problem. Followed the article to the letter.
Mine are as if I have little to no fluid pressure. I haven't tried pressure bleeding yet. Bronco is on stands so I tried stalling the motor in first gear(granny gear) 4wd high with the brakes and I was unable to stall it. My rear brakes are adjusted up to just drag enough to prevent pulling off the drums. All new stainless hard lines. Do have fluid flow no pressure.

We are not alone I have seen others complain while others celebrate victory. My next step after pressure bleeding will be another master cylinder. Cardone is not know for a good record on rebuilds.

One thing mentioned to me was when bench bleeding MC do NOT fully depress it. This could cause seal damage. Almost tired of messing with. If this doesn't work am going to move up to late model Super Duty Hydroboost with rear discs. I have a new OEM hydroboost and MC,.I need rear disc parts. Was.going to wait on that upgrade till I upgraded to a Sterling axle.

I bought my booster and MC from Napa. Not sure what brand it is. But there is no air in the system now since I had it pressure bled. So I'm at a loss at what the problem could be. The pedal feels normal now, not soft (air) or hard (no vacuum). There is 15" of vacuum at idle and was told that was enough. I have adjusted the push rod also. I have run out of possible solutions.

Some have gutted the prop valve with success but why should you have to.

Pushrod should extend one inch past the mc mounting face of the booster.

I am at a loss and left with scratching my ass. Usually brake systems don't give me.any problems to diagnosis. If it was really worth the trouble I would do.the math to figure how much pressure and volume there should be.

Your new rotors and brake pad will not help braking effort. Those are only good for preventing overheating and brake fade.

If I gut the prop valve, it would effect the rear drum brakes and not in a good way. So I'm not sure that's the answer. The pushrod length for a F150 is .931"-.946". For a F350 it's longer than that, but still just short of an inch. I have mine adjusted to the F150 length.