not enough vacuum
Thread Starter
Teching In
Joined: Jun 2007
Posts: 4
Likes: 0
From: West Atlanta Ga
not enough vacuum I have a 4.8 in my 71 chevelle,and I was wondering how much vacuum my engine should make at idle.Its only making 14-15 in. now. My power brakes have a very hard pedal. I have replaced the booster m/c and rebuilt all four corners.And no help.I have heard that I need 18 in of vacuum for the booster is that true and is this my problem?
Are you using a stock replacement master cylinder? What about the brakes, did you convert to power or disks at the same time?
I'm just wondering if you have mismatched components causing the issue.
How did everything work when you pulled it apart?
I'm just wondering if you have mismatched components causing the issue.
How did everything work when you pulled it apart?
What other problem are you having, car not slowing down beside the hard pedal ?.. do you have the rod connected up to correct hole in the pedal arm.. is there multiple holes on the pedal to use (upper/lower)? is the rod the correct one.
Did you convert from manual to power ? many times manual and power brake parts are not the some and must be swapped as a whole system.
You can always try to add a extra Vacuum tank to assist. or go to hydro assist.
But I would think that many of the older cars as yours and mine are, really only ran with about 14-16 in Hg of vacuum unlike the new engines that can produce up to 20in HG.
Did you convert from manual to power ? many times manual and power brake parts are not the some and must be swapped as a whole system.
You can always try to add a extra Vacuum tank to assist. or go to hydro assist.
But I would think that many of the older cars as yours and mine are, really only ran with about 14-16 in Hg of vacuum unlike the new engines that can produce up to 20in HG.
Thread Starter
Teching In
Joined: Jun 2007
Posts: 4
Likes: 0
From: West Atlanta Ga
All of the parts are stock. Original power front disk rear drum brake car. When it came apart it had a very soft pedal, leaking m/c, and proportioning valve leaking also, along with the rear cylinders. Makes it hard to compare.
Is the booster new and the check valve good? I would guess that one of the two are bad. You've got more than enough vacum to run the brakes. I've run power brakes down to 10 inches of vacum with no problems.
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On my Jimmy my vacuum gauge reads about 10" at idle and my brakes work great. I also have a vacuum reservoir. I got the reservoir because when I first got it running when I stepped on the brakes motor would die. Turned out to be the tune among other things not related to vacuum supple. To me it sounds like the booster isnt getting the vacuum or its bad.
