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Old 11-19-2003, 05:49 PM   #1 (permalink)
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I finally got a 1970 Bendix power booster and a 69 power brake pedal and I was ready to make my convertible a power brake car. My local mechanic and friend advised that I let him do a vacuum check to insure that I had enough vacuum to operate the brakes. He said that 15 - 16 inches of vacuum is recommended to operate the thing properly and, by the sound of my cam lope, he doubted I'd meet that requirement. It only had 10 inches of vacuum when we tested it. We checked with a local "brake guru" and he said no dice. We talked about using a reservoir to provide vacuum for the booster but we feared that it would only provide enough for one or two pumps. The mechanic did go through my brakes and corrected some "minor" problems so that the rear brakes actually work now, so I'm feeling much better about the stopping power of my ride but, has anyone else had this problem and what solutions did you either consider or use?
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Old 11-19-2003, 05:57 PM   #2 (permalink)
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You could go to an electric vacuum, my mech did not want to go to large on my cam because of this.
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Old 11-19-2003, 06:08 PM   #3 (permalink)
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Hello.If your motor is only pulling 10 inches of vacuum,then you have other issues that will need to be addressed,like that bullet hole in your intake manifold. ::
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Old 11-19-2003, 06:14 PM   #4 (permalink)
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I bought a reserve tank at PAW for around $50.
Made by crane Cams. It will hold and store highest vacuum the source you hook it up to acheeves.
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Old 11-19-2003, 06:14 PM   #5 (permalink)
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Old 11-19-2003, 06:37 PM   #6 (permalink)
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I'm assuming automatic....

Add 100 rpm to your in-gear idle speed. I'm assuming you have a looser than stock converter. Tune idle mixture for highest vacuum and time for highest advance short of detonation under power.

Install vacuum reservoir mentioned by others.

You should be OK.
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Old 11-19-2003, 06:49 PM   #7 (permalink)
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Quote:
Hello.If your motor is only pulling 10 inches of vacuum,then you have other issues that will need to be addressed,like that bullet hole in your intake manifold. ::
Veronica. If your cam has an extended duration that helps provide more efficient breathing by opening the valves longer the engine may not make as much vacuum as a result. The valves just spend too much time open to generate enough vacuum to operate accessories like tilt away wheels and power brakes. No bullet holes or vacuum leaks necessary. It's just the result of my desire to go fast getting in the way of drivablility. ::
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Old 11-19-2003, 09:23 PM   #8 (permalink)
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What are your cam specs?
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Old 11-19-2003, 09:41 PM   #9 (permalink)
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Stainless Steel Brakes offers an electric vacuum pump for around $275.00 to combat this type of problem. I believe the new 99-up Ford deisels use an electric pump also.
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Old 11-19-2003, 10:57 PM   #10 (permalink)
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Ragtop, one or two good pumps on the pedal is all you need to stop the car....the vacuum reservoir recuperates pretty quickly..

Them vacuum pumps are pricey....but are the answer.
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Old 11-20-2003, 06:51 AM   #11 (permalink)
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I've got only 10.5 inches of vacuum on mine due to the cam. I'm running the stock power disc brakes and they work. They just don't work overly well, I'm going to install a vacuum resevior on mine this winter but your brakes willl work. My main problem has been at the track, trying to stall the car up to launch.
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Old 11-20-2003, 11:21 AM   #12 (permalink)
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Suggestion:
When you clean the engine just prior to rolling in, hold the brake pedal down and just release enough to ease the car in. The booster check valve will hold vacuum and provide you with assist as you stage.

BTW, I stage the race car with an old single MC, single puck Airhearts and stock small Mustang drums and no power assist, most recently at 3.5K rpm but, with the earlier engines, at 4.5-5K. Never a bit of trouble. The single pucks, with the line-loc set, even held the car fine in the water box, even if I pulled to the front of it.

Perhaps, if you elaborated on your staging difficulties, I could help further....
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Old 11-20-2003, 12:37 PM   #13 (permalink)
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The cam is a single profile with a duration @ .050 of 224/224 and lift of .500/.500. The lobe separation is 110*.
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Old 11-20-2003, 09:08 PM   #14 (permalink)
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Pat, I have roughly a 3400 stall converter and need to launch at 2000 to get my good 60' time. I have no problem holding the car with the line lock for the burnout but I seem to run out of vacuum after the burnout. Probably from wide open throttle, right? I have tried idling to the prestage light after the burnout to build vacuum and then holding the brake and letting the car push into the stage light. Are you saying once I hit the brake and don't let go it will hold at the same force indefinitely? Even using this method I've never got the stall past 2000 rpm's before the car would roll again. Maybe that's all I should get with that converter? What do you think? I've got front disk and rear drum.
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Old 11-20-2003, 11:26 PM   #15 (permalink)
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I make really good vacuum in the water box, generally around 18" (I do high gear burnouts), until I start to roll out and the tires begin to recover, at which point I let off (to save the converter sprag). The only reason I had a vacuum guage in the car was to determine how the engine was running, in concert with the tachometer. Back when I raced, most cars were uncorked (mine has a full exhaust system) so I could hardly hear myself think much less hear my engine well.

In your case, I'd do the burnout, which should provide you with decent vacuum, and then, after rolling out and stopping the first time to clean out the engine (if necessary), keep your foot on the brake and proceed forward to pre-stage. I've never had the need to do dry hops, but doing that (if necessary for you) could cause some vacuum loss. By keeping your foot down on the pedal, you keep the same vacuum working for you, rather than losing it when lifting. The way the booster check valve works is to isolate the booster diaphragm chamber from the outside whenever there is less vacuum outside.

As for not being able to hold the car, I'm at a loss. I don't have nearly the braking capacity you have, no power brakes and tires far bigger (no slippage) but I can never recall the brakes not holding at the starting line, even when I'd run the engine up to 4-4.5K. Granted that was with a 289/302 which didn't make the torque of the bigger engines, but it still launched impressively *G* With the W and a 10" converter, I ran it up to around 3.2K against the brakes with no problems whatsoever. I never even thought about that stuff until preparing this post.

I do know, from my brake light racing episodes at the big end, I could lock up the Airhearts with the stock MC I was using. Perhaps my friction materials are softer or something. I know for sure everything is still asbestos; yeah, it's that old...*G*

You might try having the drums machined slightly and having the rotors ground....maybe a change in the friction surface might help.

If you set the line loc, can you slide the front tires? You should be able too, with slicks on the back.

Try a few things and get back to us.
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