What is enough vacuum for power brakes?

Brakes for your Classic Mopar

  1. mopar nut

    mopar nut Well-Known Member

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    Restoring a 71 Dodge Demon. It has a 440 Big Block 4 Speed. It has the Kelsey Hayes 4 Piston calipers up front and I swapped out the rear drums and shoes for 10 inch. I have replaced the Proportioning valve and all the metal and rubber lines calipers and shoes and hardware.

    I had a hard time bleeding the brakes (Vacuum bled, gravity, 2 man) I finally was able to get a good pedal but when we road tested the car it seems to stop normally but if I try a panic stop or press the brake pedal a couple times the pedal feels hard and the car will keep rolling.

    The engine has been rebuilt and was bored 40 over, crank 20 under, flat top pistons w/valve reliefs and a Comp Cams 292H Grind. Duration 290 (@.050 lift 244) Valve Lift 0.501

    My question
    Is this cam creating enough vacuum to the brake booster?
    If not besides going to manual disc brakes is there any way to correct this?

    I am running the Single Diaphragm Booster but starting to wonder about the Master Cylinder I was sold
     
    Last edited: Dec 8, 2020
  2. MOPAROFFICIAL

    MOPAROFFICIAL FABO Gold Member FABO Gold Member

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    Without measuring ..there is no way to know exactly how much vac...but that cam screams about 8" at idle installed in its sweet spot.
     
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    • Kendog 170

      Kendog 170 Let the boy go !

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      • dano

        dano Evil Handy Man

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        If your keeping the cam and want a power brake system you may need the vacuum canister or move to a hydroboost system.
         
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        • yellow rose

          yellow rose Overnight Sensation FABO Gold Member

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          You’ll need a vacuum canister or better yet, convert it to manual brakes and be done with it.
           
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          • volunteer

            volunteer Well-Known Member

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            Even a cam that gives you only 8 or 9" vac. at idle (900 rpm's) will still operate the booster and, it should be well above that value when driving or coasting. I suggest you connect a vac. gauge and get a reading at slow idle, fast idle and maybe above 2000. Depending on final drive ratio and where you do most driving, any cam over the factory (H.P.) grind would not be my own choice. Nothing quite like getting a 'chirp' in all four gears - - and 15" vac. at idle.
             
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            • 67Dart273

              67Dart273 FABO Gold Member FABO Gold Member

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              If it turns out "you need more" (or would that be "less" LOLOL) my 04 GMC has an electric vacuum booster pump hooked to the brake booster. I believe it's pressure (vacuum) operated
               
            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              I ran that cam with power brakes and a 4-speed, in my 367, no problem, BUT NOT with the dual diaphragm.
              I had to swap in a single diaphragm .

              The thing is this;

              The booster has a minimum vacuum it likes to see and as long as your engine is creating it and your check-valve is trapping it, the booster will work.
              With a 4-speed and downshifting, there was never a time when the vacuum fell too low.
              The only time I had problems was when sitting at a light, on an incline for two minutes; the vacuum was not being replenished and I had to continuously increase my foot pressure not to roll backwards. I installed a roll-control.
              But eventually, I took that booster apart and not finding anything wrong with it, and having a booster from a 73Dart, I decided to try it. It worked well enough that 20 years later it is still on there.
              To charge it up in the morning, I just blip the throttle a couple of times before backing out of the carport.
              I swapped out to a 15/16 M/C, so even if I loose a lil assist, it's no big thing. Plus this M/C has excellent moduability?, that is to say; it is easy to modulate.
               
            • mopar nut

              mopar nut Well-Known Member

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              Went out and put a gauge on it and its showing 8" at idle like you said. 16 " at around 2000 RPM
               
            • mopar nut

              mopar nut Well-Known Member

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              Manual brakes is what I was thinking, Any idea what I would need to replace/keep to do this switch?
               
            • Kendog 170

              Kendog 170 Let the boy go !

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            • mopar nut

              mopar nut Well-Known Member

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              Went out and put a gauge on it and it fluctuates between 8" and 9" at idle about 850 RPM. And raised RPM's to 2000 and it was fluctuating between 15" and 16".
              Really liked the sound of this cam setup and driving to local cruises and gettogether's would be the most - maybe take it one time down the 1/4 mile to see what it does.
               
            • mopar nut

              mopar nut Well-Known Member

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            • yellow rose

              yellow rose Overnight Sensation FABO Gold Member

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              I looked at those links and the biggest detail isn’t given, and that the bore diameter of the master cylinder. It should be 15/16 or if you like a bit firmer pedal feel 1 inch. I’ve never gone bigger than 1 inch.

              The other thing is you need a different pushrod. Other than the new MC and pushrod it’s pretty much a bolt on deal.
               
            • mopar nut

              mopar nut Well-Known Member

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              Thanks for the info. I would have to replace or cut/fab brake lines to fit also. With the first engine/cam I went from Manual to Power brakes so I think I have the original manual pushrod in my stash of parts somewhere gonna go out and look.
               
            • Kendog 170

              Kendog 170 Let the boy go !

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            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              Here's a test for you;
              With the engine off, and a vacuum gauge plumbed to where you can see it from the driver's seat;
              Pump your booster down, to less than 7 or 8 inches.
              Next, apply the brake pedal with modest pressure; then start the engine and let it idle, while watching the vacuum gauge.
              IF/when the pedal starts falling, check your gauge. If the pedal does not fall, then it is not being evacuated, so rev the engine up an inch or two and wait. It only takes 5 to maybe 10 or so seconds to evacuate the chamber. Continue in this way until the pedal drops.; then immediately check your vacuum gauge, and your rpm.

              What you see on the gauge is the minimum vacuum required to operate this particular booster, and the rpm is the minimum rpm to get the minimum vacuum. Write this down.
              If you have a dual diaphragm booster I can almost promise you the minimum vacuum will be up around 12 inches.
              If you have a bigger single diaphragm, probably 9/10ish
              Now, with your foot still on the brake,read your vacuum and shut the engine off while not moving your brake-pedal. How long does it take before the vacuum falls to the minimum? If it drops like a rock, you have a bad booster, abort the test. But if it holds for more than one typical braking application of say 10 or 15 seconds, then you are good to go.
              If you aborted the test; restart the engine and get the booster charged. This time remove your foot from the pedal, read the gauge, then shut the engine off, while watching the vacuum gauge. Ideally, your vacuum gauge reading should hold for hours. But if it drops like a rock again, then either; the checkvalve is faulty, or the diaphragm is ruptured, or your control valve is leaking, or your pushrod is too long. The rate of loss can point to the faulty member, but since we have no baseline, it is as good as impossible to use for this test.
              Usually if the diaphragm is bad or the C-valve is faulty, then you will hear the hissing up under the dash.
              To check the checkvalve, you can have a helper pinch the line at high vacuum, and watch the gauge, as you shut off the engine. But this only works if it is the only faulty part of the system.
              Pushrod length is harder to set.
              The length has to be synchronized to your driving style.
              If it is only a little too long, then the booster will give instant assist, which is extremely annoying if you tend to brake late.
              If it is only a little too short, then boost is late, and it takes more pedal to get assist. and you get to thinking it might not be working so good.
              If it is a lot too long, then you could shatter your control valve on a panic-stop.
              If it is a lot too short, boost is very late and quite whimpy.
              Once you get it into the sweetspot, 1 turn longer or shorter is a lot. Getting to the sweetspot is a lil harder. There is a procedure in the FSM to get you started.

              If you swap M/C bores, you will need to resynchronize for the change in pedal stroke that will occur.
               
              Last edited: Dec 8, 2020
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              • SLOPAR72

                SLOPAR72 Well-Known Member

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                In regards to the vacuum amount... Having done conversions of all kinds in regards to brakes, motors, etc the sweet number I have found is 14. That's a sufficient signal to the booster under all conditions. I can't speak for your setup however if I get 14 inches on the gauge it will have good brake assist assuming the unit is in working order...

                JW
                 
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