Power brakes and engine power - Vintage Mustang Forums
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post #1 of 19 (permalink) Old 08-24-2017, 05:39 PM Thread Starter
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Power brakes and engine power

Will power brakes rob engine power? I wouldn't think so, since they only use vacuum, but after I installed power brakes, my car seems to have less power. Maybe it's all in my head. Maybe something else has gone wrong. It may also be in my head, but the engine sounds different. Different idle, and different when revved.
The only thing I changed was the brakes, so I'm really scratching my head here.


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post #2 of 19 (permalink) Old 08-24-2017, 05:51 PM
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power brakes typically operate off manifold vacuum to the booster, so you shouldn't be losing eng power, but you might have a vacuum leak, or possibly dragging a brake if you haven't made brake adjustments yet.
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post #3 of 19 (permalink) Old 08-24-2017, 05:56 PM Thread Starter
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power brakes typically operate off manifold vacuum to the booster, so you shouldn't be losing eng power, but you might have a vacuum leak, or possibly dragging a brake if you haven't made brake adjustments yet.


I'm pulling vacuum of the rear port of my edelbrock carb. Which is lower than it should be. I'm reading 5". Vacuum leak is possible, but I've sprayed carb cleaner around and didn't notice any crazy surges in RPM.
I've also put front disks on with mustang steves kit and I don't think they're dragging. The change in engine sound coupled with what feels like a lack of power is what has me concerned.
Also, the motor will bog slightly when I press the brakes while sitting in park. Is this an indication of anything?


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post #4 of 19 (permalink) Old 08-24-2017, 07:48 PM
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Could be the booster that's leaking. Search YouTube for videos on how to check for booster leak.
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post #5 of 19 (permalink) Old 08-25-2017, 12:35 AM
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Booster Leak

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Originally Posted by qbui View Post
Could be the booster that's leaking. Search YouTube for videos on how to check for booster leak.
+1 on the booster leak, particularly since you're feeling it when stepping on the brakes at a stop sign.
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post #6 of 19 (permalink) Old 08-25-2017, 01:26 AM
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5" of vacuum is not enough. Power brake boosters require at least 15" of vacuum to operate. Try pulling it off the manifold and see if that changes anything.


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post #7 of 19 (permalink) Old 08-25-2017, 08:40 AM
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Disconnect it and plug the hose, then you will know if you are nutz or not - context sensitive of course...
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post #8 of 19 (permalink) Old 08-25-2017, 08:41 AM Thread Starter
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Originally Posted by Kelly_H View Post
5" of vacuum is not enough. Power brake boosters require at least 15" of vacuum to operate. Try pulling it off the manifold and see if that changes anything.


Thats what is also odd. Brakes seem to work great


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post #9 of 19 (permalink) Old 08-25-2017, 09:06 AM
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Quote:
Originally Posted by Dads68Mustang View Post
I'm pulling vacuum of the rear port of my edelbrock carb. Which is lower than it should be. I'm reading 5". Vacuum leak is possible, but I've sprayed carb cleaner around and didn't notice any crazy surges in RPM.
I've also put front disks on with mustang steves kit and I don't think they're dragging. The change in engine sound coupled with what feels like a lack of power is what has me concerned.
Also, the motor will bog slightly when I press the brakes while sitting in park. Is this an indication of anything?


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When are you reading 5" of vacuum? Remember when you step on the brake the engine vacuum will rise dramatically since you've just closed the throttle. Five inches seems very low, but it is probably well above 10" when you're moving and let the throttle close.
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post #10 of 19 (permalink) Old 08-25-2017, 09:50 AM Thread Starter
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When are you reading 5" of vacuum? Remember when you step on the brake the engine vacuum will rise dramatically since you've just closed the throttle. Five inches seems very low, but it is probably well above 10" when you're moving and let the throttle close.


5" is at idle around 700 rpm in park from the rear port on the carb


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post #11 of 19 (permalink) Old 08-25-2017, 10:07 AM
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5" is at idle around 700 rpm in park from the rear port on the carb


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Something is amiss. As was stated (Kelly), your engine idle vac reading should be in the upper teens to properly fund your brake system. How and where are you getting the vac reading? Pull you booster vac from the manifold. Leave the carb base for the PCV system and the C4 modulator (if an auto tranny).

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post #12 of 19 (permalink) Old 08-25-2017, 07:25 PM Thread Starter
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So I pulled the power booster hose and plugged the hole. Same deal.
On another note, the car recently intermittently will not crank when I turn the key. I won't do anything like the battery is completely dead (which it is not). I'm starting to think that my loss of power and rougher idle possibly has to do with ignition. This is obviously just a hunch. I was thinking the start issue may be a loose wire or something simple, but I'm not sure. Could this be the pertronix or coil, or something else?


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post #13 of 19 (permalink) Old 08-27-2017, 12:17 AM
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If it won't crank when you turn the key to "START" then the possibilities are....

1. Ignition switch
2. Starter Solenoid
3. Neutral Safety Switch
4. Wiring between them...

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post #14 of 19 (permalink) Old 08-27-2017, 07:30 AM Thread Starter
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I think I've solved it. At least the idle/power problem. I sprayed carb cleaner through the carb, revved the engine a few times and like a switch was flipped, it started idling normally again. Maybe I had something caught up in there.


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post #15 of 19 (permalink) Old 08-27-2017, 07:44 PM Thread Starter
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I think I got a little ahead of myself. My problem seemed to rear its ugly head again today while driving through town. I goosed it a little bit coming off a side street. Pulled up to a stop light and stalled out. Idle quality was shot again. I kept it running by maintaining throttle and pulled into a parking lot. Revved it up a few times, and she ran normal again. Has me thinking fuel/fuel filter maybe. Any ideas out there?


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