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Hot Rods Vacuum reducer?

Discussion in 'The Hokey Ass Message Board' started by Ragtop, Dec 19, 2015.

  1. Ragtop
    Joined: Nov 17, 2001
    Posts: 1,260


    Anyone know of a fitting that reduces the vacuum to a brake booster? I've heard Gm used them but can't seem to find anything like that. Whoever had my Chevy 11 before me put power brakes on it and the front wheels lock right up as soon as you barely touch the brakes.
  2. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,833

    V8 Bob

    If you need power assist, you don't want to limit the vacuum. Maybe you're using a manual pedal ratio? Try braking with the booster disabled. Front drums or discs?
  3. 54fierro
    Joined: Jul 6, 2006
    Posts: 493

    from san diego

    I suspect they added the booster to the manual brake cylinder. The manual brake cylinders were usually 1" bore on the manual brakes and 1 1/8" for power brakes on those year gm.

    Your issue is no going to be with the vacuum
  4. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,617

    Rusty O'Toole

    They used to use a different brake pedal to reduce the leverage on power brakes too. If someone put a booster on you need to make other changes (master and pedal).

  5. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 28,937

    Jalopy Joker

    so, what is the vacuum reading from the motor? post some pics of what you are working on
  6. little red 50
    Joined: Feb 19, 2011
    Posts: 212

    little red 50

    Do you have a combination valve? If not it will cause that.
  7. Ragtop
    Joined: Nov 17, 2001
    Posts: 1,260


    Don't know the MC bore but it looks like it was a kit. New MC and booster as well as front discs with the original small rear drums. It does have a combination valve with an anodized bracket that fastens to the MC/ Booster as if it came in a package deal. I'll post some pics. Can't tell you what the engine vacuum is right now because it's out of the car. It will be the 406 out of my T coupe and has a hydraulic cam that's not real radical, although it did need a converter. The T never had anything but a vacuum advance on it so I don't think I ever checked it.
    Joined: Jul 20, 2006
    Posts: 2,422


    Boosters always use vacuum direct from the manifold, never reduced in any fashion.
  9. Joe H
    Joined: Feb 10, 2008
    Posts: 1,202

    Joe H

    The ideal vacuum reducers is a big thumper camshaft!
    54fierro, little red 50 and belair like this.
  10. belair
    Joined: Jul 10, 2006
    Posts: 8,893


    Don't think vacuum is your problem.
  11. skipperman
    Joined: Dec 24, 2002
    Posts: 1,837


    Check with the company you purchased the kit from .... I know of 3 guys that had BAD prop valves brand new from the manufacturer .... just a possibility ...
  12. DdoubleD
    Joined: Nov 18, 2009
    Posts: 225

    from Michigan

    I had a similar issue. When I installed a power booster on a project car I built, the reaction disc (see pic) fell out and I didn't know it. The brakes way over reacted with the slightest input. Took me till I got to my wits end and bought a new booster, that's when I found the rubber reaction disc on my old booster was missing. Gave my old booster a good shake and guess what fell out. Some of the above posts need to be considered also. I'm just throwing it out there because it drove me crazy for a long time. Good luck!

    Attached Files:

  13. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,682

    Bruce Lancaster
    Member Emeritus

    The '60's type GM cars changed pedal ration for power...I researched this out a bit when I went to power discs on old Novas. These and others of the period simply had two holes drilled in the pedal and I simply moved the pushrod clevis from the standard hole to the power hole for that part of the conversion (and, irrelevant here, added a little factory tab that maintained the proper push on the brakelight switch). This was on '68-72 Novas (the '68 being a legit Chevy II) and I don't know if its the same on earlier ones. If it is, I happen to know that the little tab for brakelight can be made from a cat food can...
    I also swapped the MC for a disc one, don't remember bore specs. I actually used Seville parts there since I was headed for 4 wheel discs.

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