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Old 12-01-2007, 04:27 PM
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vacuum source & line

I have a '56 F100 powered by 283 Chevy, mild cam with a CPP Power Brake kit w/ Prop. valve. Disc up front on my MII and Drum on my 9". I think my brakes lack the power assist that they should have therefore; I have 2 Ques :
1) Which is a better source for the vacuum, the back of my Carb or the Intake?
2) Of what material should the vacuum line be.....presently just have a 3/8" gas line hose. (It's about 4 foot long due to the Booster/Master located under cab.)

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Old 12-01-2007, 11:17 PM
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heya bud, not an expert BUT , the vac take off should be the same when hooked from the carby base or direct to intake pressure , get a vac gauge and check - it should be up around 18-plus except for when you open the throttle, see what she measures up to and then u can start figuring why its low or whatever...could be the booster is sluggish or whoknows , start with the vac gauge , keep in touch with ya news cya Rix66
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Old 12-01-2007, 11:49 PM
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Rix66 is right about the carb base or manifold, either port should deliver enough vacuum, anything above 16" you should see some assist especially at speed. Don't forget, usually you are creating more vacuum at speed when you take your foot off the gas, throttle plates close, vacuum goes up.

Check your hose, it may be collapsing. I use either 3/8" vacuum hose, (thick wall stuff, won't collapse), or 3/8 steel tube, with short lengths of hose to connect at the ends.

What size booster are you using? A 7" single diaphram booster will only give minimal assist, especially at lower vacuum levels..

You can check the booster by shutting the motor, pump the brake pedal a few times to allow the booster to drain it's vacuum,(how can you drain something that isn't there?hhahaha).

Then put your foot on the brake pedal and apply some constant pressure, just till you feel resistance. Then start the motor...if you feel the pedal get easier to push, and "fall" under your foots pressure as the motor pulls a vacuum on the booster,, then the booster is probably working.


You could have too big a master cylinder, wrong pedal ratio or you might just have glazed pads or too small a rotors, etc,etc. There are many other things that could cause poor brakes.

HTH, mikey
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Old 12-02-2007, 08:18 AM
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Also make sure that there is the one-way vacuum check valve where the fitting is on the vacuum booster. If it is there, it should hold vacuum in the booster after the motor is shut off.

I'd take the truck on a short run in normal driving; then stop and feel all 4 brakes for normal temps, and be looking for "cold" brakes on one axle. Sometimes you can have a bleeding problem or valve problem and you are only stopping on one pair of brakes.....been there, done that
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