Vacuum Brake 1932-1948 Ford

BUBBAS IGNITION

BUBBAS IGNITION

Well-known member
I get questions each and every week regarding the vacuum brake used on the flathead engines.
Heres my very best description of operation...

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FORD VACUUM BRAKE DISTRIBUTORS
1935-48 Ford , Lincoln and Mercury.

These units act as a conventional distributor employing a centrifugal governor advance.
The separate vacuum brake consists of a plunger or piston which is held against the braking surface of the governor plate by a spring of adjustable tension. As the rapidity of combustion is dependent upon the degree of compression, the need for a retarded spark for quick acceleration or power is not dependant on engine speed.
The actual requirements for retarded spark of any speeed are when the vacuum in the carb throat is extremely low. As an example , if the car is traveling at a speed of 20 to 25 MPH, the throttle valve would be partially open and would restrict the passage of air into the manifold which would result in a comparatively high vacuum in the intake manifold causing the air in the suction line to be drawn into the manifold.
The suction draws the vacuum brake piston upward, compressing the vacuum brake operating spring. When the brake piston is in this position, the brake piston is in this position , the brake is inoperative and the timing is automatically advanced by the centrifugal governor weights.
However , if the throttle is fully opened suddenly, the restriction to the air entering through the carburetor throat would be removed and the vacuum in the intake would immediately drop. The operating spring then pushes the piston downward against the governor plate , retarding the spark.
As the engine speed increases to the speed required by the throttle valve position, its increased demand for air again causes a partial vacuum to be formed and the air is again drawn from the suction line and the vacuum brake is again inoperative.
The vacuum brake setting can be accurately set with the distributor is off the car by using a dostributor machine. However if this equipment is not available , the vacuum brake may be set as follows.
1-Inspect the brake piston for any signs of binding in the distributor body. Apply a few drops of oil to the piston.
2-Set the vacuum brake so that the engine pings under load.
3-Adjust until the ping ( spark knock) is removed. Avoid screwing adjusting the adjustment down more than is actually needed or the spark will not advance properly.

If satisfactory results cannot be obtained the distributor should be removed, checked and timed….

Note !
A properly adjusted vacuum brake will apply pressure against the governor when the engine is not running, allowing the spark to be slightly retarded when cranking and pulled back as soon as the engine starts allowing instant advance from the advance weights.

Discussion ?????????
 
What about the surface finish of the disc that the piston leather runs against-- rust can alter the friction and cause erratic advance depending on how much ans the percentage of the diameter covered
 
Thank you Jim, This is very clear and informative. Very similar to the operation of the SBC mod.
 
I have cleaned up the disc in the lathe using a carbide tool,the disc is really hard,the pits in the disc will soon eat up the leather,if not taken out.
 
Yep the governor wheel or disc needs to be in good shape. In most modes of operation the piston is either in contact or out of contact. Not much of a in between position...
 
What about the surface finish of the disc that the piston leather runs against--
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The edge of the disc appears to be chrome plated. I worked on one that was rusted where the leather pad had been resting; I think that the tanning acid from the leather had attacked the chrome. The leather needs a little oil on it but this one had dried out.
Used a carbide bit as Don described to smooth it out, but now the chrome is gone.
 
When Ford made these parts they must have thought they were going to last 100 years,the workman ship and quality is first class,
One of the reasons for owning an old Ford.
lawrie
 
It depends on what has been modified and how ---

Most likely you are leaving economy and possibly power go out the tailpipe wasted
 
I have a pertronix in my crab distributor and the vacuum line isn't hooked up. That is correct? Right?
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Whether or not the vacuum brake is operational or not has no relationship to how the spark is generated. The vacuum should be connected unless there is some other overriding consideration, which would be unusual.
 
packyusmc said:
I have a pertronix in my crab distributor and the vacuum line isn't hooked up. That is correct? Right?
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I also have a Petronics unit in my distributer, but after all the reading of posts here, I hooked up the vacuum and put in a piston...

Can't tell the difference. The engine never did ping, I checked the timing with a light- it's good, the advance works...

The idle has improved, and it starts cold better...
 
ALL early Fords should run the piston, leather,spring and adjustment nut no matter what system is used in the distributor.
They should have a slight drag especially during startup to help stablize the spark . Whether the vacuum is hooked up is a secondary discussion as its pretty hard to have a flathead ping or spark knock on todays fuel. If a ping is heard then it should be adjusted to help the tip in ping as needed.

Run the vacuum brake ! If not hooked up back off the adjustment to just have a slight drag on the retard wheel..
 
One thing you must remember is: Converting any points distributor to electronic, has no bearing on the spark curve. The stock Ford distributor in proper working condition, is the best ignition system you can buy. Nothing on the aftermarket can equal it. Now tuning it is another story.
 
Discussion ?????????

The first thing I noticed in your illustration was that your distributor is running backwards!!! lol

Apart from this glaringly obvious oversight, well done, a most informative description.
 
Question: The pad on my brake piston is GONE. What should I use to replace it? Leather belt of what thickness?
 
The originals were 16 OZ. leather, which was about 1/4" thick. A couple of years ago, I bought a chunk of 16 OZ. leather and a 5/8" hollow punch and made my own, I think I saw one floating around my bench the other day. PM me your address and I'll send it to you.
 
Thanks Tubman
I will try to find some 16oz around here. I have hollow punches.
besides I can't find how to PM ya.
Will find out if I am unsucessful finding the leather. shouldn't be too hard.
Shoe or Harness shop maybe.
 
16 OZ is quite hard to find. In reality, you can get by with thinner leather; just as long as it is thick enough to sit reasonably higher than the rim on the piston.
 
tubman said:
16 OZ is quite hard to find. In reality, you can get by with thinner leather; just as long as it is thick enough to sit reasonably higher than the rim on the piston.
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Actually, check your local shoe-repair shop. I bought a leather heel pad for a dollar or two and used that. It's about a quarter-inch thick and one was enough to make perhaps 10 vacuum-brake inserts.
 
tubadon40 said:
...besides I can't find how to PM ya.
Click to expand...

Just right click on the name to the left in a post (the underlined one) and you get a drop-down menu. Select "Send A Private Message To...". From there it is very similar to an email, enter a title, your message and the send it.
 
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