Hey y’all…a few questions and then I will explain why I asked them. The car has front discs and drum rears. There is no proportioning valve in the system. The Master cylinder is bolted on the front of a 7” power booster, hanging style pedal assembly. Question 1: Should it be possible to press the brake pedal hard enough and fast enough (think: panic stop) to lock up the front OR rear wheels? Question 2: Where is the best place to draw the power booster’s vacuum from? (is the rear base of the carb a good location?) Details: Car is a 1950 Ford Crestliner (my high school car - owned since 1972) with a 1965 Chevy 396 Big Block + a 700r4 trans. It has a universal hanging pedal assembly with a 7” booster on it. The booster is plumbed into the back of the carb spacer, for its vacuum. The brakes have never been able to “squeal the tires and stand it on its nose”. You can mash the pedal hard and the car will ‘drag’ to a stop. Nothing locks up. I always thought it was because of the lack of a proportioning valve, but an experience with my daily driver opened my eyes to a different possibility. I was pulling into my drive last week and pushed the brake pedal and the car just kept going, so I stood on it and got it to slow down ( “Drag”) to a stop. At the shop yesterday, they said the power booster had gone out, so I had no power brakes. So, now I am wondering if my booster in the ’50 is not activating….that was the reason for the question about the best place to pull booster vacuum. Open to any thoughts or suggestions…
Do you have a check valve in the vacuum line between the booster and the intake manifold? How much manifold vacuum does it pull at idle?
What diameter front discs, what calipers, what master cylinder and diameter, how much idle vacuum, what is the pedal ratio, and last rear drum brake diameter? A lot to ask but each item makes a difference. I have 11” front discs with 4 piston calipers, 11” rear drums, 7/8” MC and no power booster and can lock up all 4 in a panic and needed to do it twice. 3600# car.
You can check the yes/no function of the booster by disconnecting its vacuum line at the source and plugging the source port. The booster may still work for a few pedal presses until the booster vacuum is depleted, but then it’s out! John
The first thing I'd change is your vacuum source. The OEMs almost without exception connect directly to the intake manifold. The restricted passages in a carb or carb spacer IMO don't allow enough air flow to allow full proper operation of the booster. Yeah, it's a vacuum but it's not a perfect vacuum, there's still air in there and that air has to exhaust out of the booster when you apply the brakes. Second, a proportioning valve may or may not be needed. You want the front brakes to lock first, if the rears lock first the car will swap ends, that's when you need the valve. I think a lot of them are installed where they're not needed.
You should be able to lock up the brakes with the engine off [or no vacuum] ,it will just be difficult that's all. You don't need a proportioning valve [yet!] They are only needed to stop the rear wheels locking up with extreme pedal pressure .[Oh f*** moments] If neither front or rear locks up , chances are the balance/bias is OK[ish] Try installing a smaller bore m/c on it. It would really help if we knew what bore m/c you have, as well as calipers ,pedal ratio etc. Up until we know these, we can only guess and suggest replacing everything one component at a time
Manifold vacuum. I think most if not all aftermarket 7" boosters are crap (single or dual diaphragm?). They do little more than get in the way. Not saying that's your problem but...
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