It says, "post quick reply". I can't seem to post a quick reply? The 1948 and earlier engines use a "vacuum brake". That is the front mounted style distributors. Those use manifold vacuum, not ported vacuum to control a spring and piston. It does not work like any distributor vacuum advance most people are familiar with. Its not complicated just different.
You need to understand what vacuum is and what effect the throttle plate opening has on it to understand how vacuum controls the timing using a vacuum brake or a vacuum advance. When the throttle plate is closed there is a high vacuum below the throttle plate. It just like sucking on a straw (now illegal here in CA) with your finger over the end, your finger being the throttle plate, there is a strong vacuum below your finger. That would be where the "manifold vacuum" port to the vacuum brake would be located below your finger. When you release your finger more air can flow through the straw but since there is no restriction (your finger) the vacuum drops below the throttle plate even though more air is flowing. The "ported vacuum" port is located right above the throttle plate. There is no vacuum there at idle when the throttle plate is closed because very little air is flowing past it. As the throttle plate opens and air begins to flow the vacuum increases at the ported vacuum port and drops at the manifold vacuum port. It depends on which side of the butterfly the port is located on as to how much vacuum you will have.
Just for fun install a vacuum gauge in your car, hooked to intake manifold vacuum. They are the simplest of all the gauges to install. You will learn a lot by just watching the readings as you drive around.
Manufacturers controlled vacuum advance and in later years many other components by turning them on or off with vacuum or the lack of vacuum. On some components vacuum opens the device and on others the lack of vacuum opened the device. Using the different types of vacuum signals gave the engineers a wide range of when and how they could turn on and off devices. If you look at late model carburetors, 1973 to around 1985 you will see several vacuum ports. Two down low on the carb, those are your manifold vacuum ports. A large one usually on the back bottom side of the carb's base plate. That goes to your PCV valve and another small manifold vacuum port to control other smog devices. About halfway up the carb there will be a ported vacuum port and possibly a third fitting which will be your venturi vacuum port. You need to understand that each port will receive vacuum to it depending on the position of the throttle plate. Its real important that the right component is plugged onto the right port. There are so meany vacuum lines on the 1980s cars there is usually a decal under the hood shoeing you where they all go.
If you have a Stromberg 97 or the other early stromberg or Holley carbs you have no vacuum ports on the carb at all. That because the distributor's "vacuum brake" used intake manifold vacuum directly from a fitting on the manifold. If you wanted another intake manifold port to control a Columbia 2-speed rear end, Southwind heater or just a vacuum gauge for inside the car you needed to add a spacer plate under the carb (below the throttle plate) with an "intake manifold" vacuum port. I definitely not an expert on all things flathead. The later flathead carbs "might" only have a ported vacuum port for the distributor advance. If you wanted a vacuum gauge in the car you might need to drill a hole for a fitting in the intake manifold. There might be one there already?
Its not like today where there are millions of sensor combinations such as rpm, temperature, engine load, air conditioner on or off, throttle position and many-many-more sensor readings. The computer goes to its look up tables and finds the perfect timing for that specific combination of sensor readings and then adjust the timing by firing the plug, based off of the crankshaft position sensor. Super precise. That's how these new engines make so much horsepower. The computer is constantly tuning the engine for every possible situation. Back before about 1985 they could only control the timing advance by the centrifugal weights or the vacuum. They used a duel diaphragm advance so they could control vacuum with both kinds, manifold and ported. They could turn on and off the vacuum with solenoids. They could delay the vacuum signal with valves that restricted the flow. They could control the vacuum with coolant temperature switches that could block the vacuum flow. You want to stay far away from any of those late 1970s and on up vehicles if they carbureted. Except for maybe the Mustang GT. The rest of them all need to be crushed Just too much stuff to go wrong. Fuel injection and distributor-less engines cured all those problems. The greatest two things that ever happened to engines.
Still, I love the old stuff even if it is a pain in the butt at times. I find it more interesting to see what the engineers could do mechanically to control things. I think you had to be a lot smarter to figure out the mechanical ways of controlling stuff than to be a computer programmer. The stuff they came up with amazes me. I just bought a basket-case 1922 Mack Truck. The engine has roller lifters in it. The different ways they solved problems back then. In my next life I'm going to be a Mechanical Engineer or a machinist. Its not going to be a phone company mechanic ever again!!!!!! If your young, I beg you not to become a mechanic. Go get a mechanical , electrical or some kind of degree that lets you create stuff.
There are some good description on the "vacuum brake" on this ford barn thread.
https://www.jalopyjournal.com/forum/...-brake.766874/
Its really important for everybody to understand vacuum readings when working on cars.
If you really want to be a "gear-head" you need to know all about vacuum to work on carburetors, distributors and diagnose mechanical problems such as bad valves. You have so much good information available online, great YouTube training videos. Get yourself a combination handheld vacuum pump, vacuum gauge and an "adjustable" timing light. Make sure it is adjustable! Then play with them. The best way to learn this stuff is hands-on . You can read forever, and be book smart, pass every automotive test and not be able to fix a damn thing. You need learn all about the electrical stuff. Staring with basic electricity, Voltage, amperage and resistance A majority of the problems posted on the Ford Barn are for fuel and electrical. Those two things, over and over are what stump most home enthusiasts. For some reason they will not take the time to understand how this stuff works. There is not much to it. You just need to be interested in it and it will be easy to understand.
The guys that say," I want something I can drive" and then they rip out the early drive train or they remove the points and install an electronic ignition. They are trying to turn the antique car into modern vehicle. Its because they did not grow up with these cars. They did not ride around for decades on points and carburetors. Never once breaking down because their dad maintained his cars properly. These new guys are afraid of their cars breaking down because they do not understand how they work. They want to just get it in and go. If you understand how your old car works you will not be afraid to drive it. If it quits you'll be able to pull over, know where to beat on it with hammer and be on your way again. If your new car quits, your getting towed to the dealer because nobody else with have the technology to fix it. If you don't think new cars break just go sit at the dealer and watch them come in on the tow trucks.