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 (Classic Chevy, Chevrolet, GMC, Ford technical articles) 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: Complete Brake Upgrade for 1978 - 1979 Ford Broncos .: Articles

 

I am in no way an expert but some of the things I am catching from your writings is your vacuum is to low and this may be the reason your brakes are soft, the booster may not be getting enough vacuum to operate properly, there is a fix using a vacuum boost can for preloading the booster, also how far down does the pedal go before it starts braking? the rod in the booster is adjustable and will allow you to shorten the throw of the pedal. As for the stalling problem, there is an idle solenoid attached to the factory carburetors throttle assembly that goes out and of course it is not made any more and is hard to find. I fixed my Idle problem by installing an Edelbrock performer intake manifold and Edelbrock 650cfm 4bbl carburetor and now she purrs like a kitten! Also your booster vacuum line should be plugged directly into your manifold and not the port on the back of the carburetor! If you replaced the vacuum hose to the booster it needs to be a special hose designed for a vacuum booster and it's not cheap hose, mine was like $30.00 for a short piece. Good luck and I'm sure other members will have some ideas!

 

It sounds like your describing a vacuum leak assuming your timing is correct and carb is adjusted and tuned (what kind of carb do you have and what is your timing set at?). One way to find it is, remove the vacuum hose from the brake booster (the other end connected to the manifold), put a rag in the carb, plug the oil cap and pcv valve holes and blow smoke into brake booster vacuum line. A good ole swisher sweet works fine or find another source. Of course the other option is to spray carb cleaner all the connections and lines and see if the engine revs. 15" of vacuum should be sufficient and as was stated earlier, make sure it is connected to the manifold.
It is possible that you have air in your master cylinder but it could also be bad. I went through three before I got one that was good.

 

MIKETAHENY--I've been suspecting a vacuum leak also. I will use the carb spray method and see if I can locate a vacuum leak. The hose from the booster is plugged into the manifold, not the carb. I eliminated the brake booster as a possibility of being the problem by disconnecting the hose (and plugging the vacuum port in the manifold) and started the engine and it still died on me.

I'm trying to think of anything I did different when I installed the new brake system....the only other thing I did was install my Ford Racing 4 1/2" tall valve covers....Let me explain why I mention this...when I bought the valve covers I had to drill my pcv and oil cap holes. Well because I wasn't paying attention to the drivers side valve cover, I ended up drilling the oil cap hole on the rear of the valve cover instead of the front. Once I figure this out I drilled the oil cap hole in the correct location, but then I had an extra hole I needed to plug. So I came up with the brilliant idea (sarcasm) of running 2 pcv valves to the carb instead of one. So I thought that maybe the extra air being dumped into the base of the carb might have something to do with the idle. So tonight I unplugged one of the pcv valves so I only had one pcv hooked up to the carb, like it's supposed to be. Then I fired the engine and tested it and it still does the same thing, just not quite as bad this time.

So long story short, the only things I have changed since it was running perfect was: install new brake system, hooked up 2 pcv valves instead of one and turn the idle down from 1000 to 800. I had it set at 1000, but it didn't want to shut off with the idle that high. It tried to keep running and coughed and spit at me, so I turned the idle down and it shuts off perfect now. Maybe I should try turning the idle up to 900 and see what happens?????

I am running a Holley Street Avenger 770. Timing is set at 14* initial.

OVERLANDEXPRESS---I also thought the vacuum was possibly the problem, which is why I ran the vacuum test. Where can you buy a vacuum boost can (if I need to go that route)? Does Autozone sell them or do I have to get them from Summit or Jegs?

The pedal goes about half way to the floor before it starts braking. I have adjusted the push rod and it's within spec. There didn't seem to be a change after I adjusted the push rod. No difference at all.

The carb is brand new and ran perfect prior to having this problem. The entire engine and carb only has 800 miles on it and ran perfect prior to installing the brakes, adding the extra pcv valve and dropping the idle from 1000 to 800.

The booster hose is plugged into the manifold, not the carb.

 

Well your description is succinct and the only other advice I could offer is look for something stupid simple that may have been overlooked.
Autozone doesn't sell the vacuum boost can.
If you are absolutely positive that you have bled all the air out the only other thing that comes to mind to check/ double check is your rear brake adjustment assuming nothing leaks.
I hope you figure it out, I'm really curious as to what the culprit is. Good luck.

 

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.

 

With the engin off...and When you bleed the brakes are you pumping them up then open the bleed port then slowly press on the brake pedal?

 

No, not pumping them first. Just pushing on the pedal, then open the bleeder valve, then close it, then lift up on the pedal and push and hold and repeat the process....

 

DOT4 absorbs air, where DOT3 absorbs moisture. DOT4 is harder to bleed because of this characteristic, this is why most people run DOT3.

Also, put a little copper anti seize or brake silicone on the threads of your bleeders when trying to bleed. Helps seal the threads and won't do anything to your seals.

It may help to pump the brakes first, it helps build pressure. Pump the brake pedal 3-5 times, hold it down, crack the bleeder, close the bleeder, let up on the pedal, repeat.

What kind of prop valve do you have?

As for your carb, even though you have it figured out, you're going to have high vacuum if you are just in park/neutral and are holding the throttle at a certain point, this is just showing you what your highest possible cruise vacuum is going to be. 14* initial is kind of a lot of timing, but if you're not pinging more power to ya.

 

That's good info, especially about the brake fluid. The reason I used DOT 4 fluid is because I was following this article about doing this conversion, which mentioned using DOT 4 fluid

file:///C:/Users/Lance/Documents/Automotive/78%20F150/Complete%20Brake%20Upgrade%20for%201978%20-%201979%20Ford%20Broncos%20.%20%20Articles.htm

Let me ask this....will using DOT 4 fluid make the brakes feel softer or have less performance (such as providing less clamping force) than using DOT 3 fluid?

I'm not using a proportioning valve. I'm using the factory setup, which is the junction block (can't remember the technical name for it; multi-function valve???).

My vacuum at idle was 15". It went to 20" at 2000 rpm. Is 15" enough vacuum to run a F350 dual diaphram power brake booster?

14* is what the engine builder I purchased my distributor from suggested I set it at. So far, no pinging. Will do my first trailer pull on Memorial Day when we go camping. Have a 29' travel trailer I'll be towing. We'll see if she pings when pulling the trailer. If I do, I have a MSD timing control setup and I can just retard the timing on the run to control the detonation.

 

It may be softer but thats only because there is air in the system. DOT4 absorbs air like DOT 3 absorbs water.

Factory setup is called a combination valve. It has a metering and a proportioning valve inside.

15" is fine, that's just at idle. When you are decelerating you will have around 22", which is when you will be using your brakes.

Trial run for timing, throw it in 4th at 20mph and mash the throttle. Do you hear pinging?

 

everything is new made me think if it has the factory manual fuel pump and the fuel bowl float is set too low wont the carb starve at low idle and kill the engine but it will keep up if revved because of the increased fuel flow? might not be a vacuum issue do you set the fuel levels and is the carb sitting level in relation to the ground? sometimes if the engine is tillted too far back it throws off the bowl float setting.

 

im not a brake expert though

 

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.