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Technical A test to determine if a brake (booster ) is working

Discussion in 'Traditional Hot Rods' started by blazedogs, Jun 16, 2020.

  1. blazedogs
    Joined: Sep 22, 2014
    Posts: 445

    blazedogs
    Member

    Plenty of vacuum to the booster & booster, check valve on booster is working, hard pedal,no brake fluid leaks, new master cyl, everything adjusted right, have played with the proportioning valve,and on & on.... I,m stumped takes 2 feet bearing down on the pedal to stop car. I have been playing with this issue for some time,driving me nuts... HELP
    Drum in the back, disk in the front, everything new, Model A w. small block chev.
    Ok here is the question right or wrong; to ( test) to see if booster is bad press down on the brake pedal,and start the car,as soon as the engine starts the pedal should drop about a inch if the booster is working ???????? T or F

    Gene in Mn
     
  2. Have you checked the brake rod to see if it's not long enough?

    Did you use a proportion valve? HRP
     
    LOU WELLS likes this.
  3. blazedogs
    Joined: Sep 22, 2014
    Posts: 445

    blazedogs
    Member

    Hot Rodder ,yup to both
     
  4. 56MercMan
    Joined: May 22, 2008
    Posts: 42

    56MercMan
    Member
    from CA

    I have a similar issue on a newer (1988) vehicle. I have replaced everything with new and I will be trying the booster next. It passes all the booster tests but I have mechanic friends and read forum posts with similar issues and they fixed it with a booster replacement. Good luck I know how frustrating it is to chase these brake gremlins... I hate throwing $ at a problem but that is what I had to do with this one, so I figure one part left might as well try it.
     
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  5. [QUOTE="
    Ok here is the question right or wrong; to ( test) to see if booster is bad press down on the brake pedal,and start the car,as soon as the engine starts the pedal should drop about a inch if the booster is working ???????? T or F

    Gene in Mn[/QUOTE]

    That test, although IMO not 100 % conclusive as to booster condition, is a quick test used in some vehicle safety inspections to give the power brake system a pass or fail.
     
    VANDENPLAS and chopped like this.
  6. What size is the bore on the master cylinder? HRP
     
  7. True. Hook up a vacuum pump and gauge to it, pump it up and see if it holds vacuum. If it will not pump up and hold vacuum the diaphragm is bad and you will need a new booster.
     
  8. blazedogs
    Joined: Sep 22, 2014
    Posts: 445

    blazedogs
    Member

    1 Inch
     
  9. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 2,355

    2OLD2FAST
    Member
    from illinois

    I know , I'm a smartass , but it tickled me when I first read the ? My first thought was , Are we so far removed from having manual brakes as standard equipment that we can't tell the difference??? Sorry , I know that doesn't help .
     
  10. blazedogs
    Joined: Sep 22, 2014
    Posts: 445

    blazedogs
    Member

    No offense taken at all I agree with such a light car as this it seems I shouldn't have to even have a booster. When I built the car it was just a after thought to include it. The car is not safe to drive now . If a kid jumped out in front of me there is no way that I could slide the car to a abrupt stop
    Even now just changed the pivot point on the brake pedal to give me more leverage.Helped a little but not much !
     
  11. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 2,355

    2OLD2FAST
    Member
    from illinois

    What are the components from ? When I did my T I used ,calipers & rear wheel cylinders , master cyl & combo valve from the same type / similar sized cars , I.e. 70's to80's ford used the same components with different sized rotors & shoes across many models , from pinto/ mustangII to fairlane, comet , Granada , mercury meteor etc, etc. At least that way you can fit the parts to the vehicle w/o a lot of variables ..
     
  12. borderboy1971
    Joined: Oct 20, 2008
    Posts: 688

    borderboy1971
    Member
    from Canada

    There really is a couple tests you can do to check the booster
    1...with engine off, press and hold the brake pedal. Start the engine. The pedal should sink a bit.
    2... Start engine and let it run a few seconds at least. Do not touch the brake yet. Shut engine off. You should be able to have 2 or 3 power assisted stops. If it fails this test it can be the booster or check valve.

    I suspect your booster is shot. But do the tests. Also take a piece of hose and hold one end to your ear, use the other end to poke around the pushrod area and listen for vacuum. I suspect the diaphragm is shot and you will hear it hissing.

    One other thing... if brake fluid is leaking from the back of the master cylinder, it will destroy the diaphram over time, so check closely that there are no signs of fluid leakage.
     
    VANDENPLAS likes this.
  13. Dirty Dug
    Joined: Jan 11, 2003
    Posts: 3,634

    Dirty Dug
    Member

    On a Model A the first thing you need to do is remove the brake booster and move on from there. The worst excuse for having something on a hot rod you don't need is that you started out with it and had to make it work. You don't need it and it's ugly.
     
    Jibs likes this.
  14. Graybeard4545
    Joined: Feb 13, 2020
    Posts: 53

    Graybeard4545
    Member
    from Maryland

    Blazedogs,
    You said everything is new? Booster and master cylinder? I ask because some boosters are shipped with a rubber dust cover on the master cylinder side as well as some master cylinders. This needs to be removed so the booster shaft to the master hits the cup in the master cylinder. I got a unit where it was in between the two and the rubber dust cover got wedged in the master cylinder. It drove me nuts for about a day. It made the peddle get stuck and hard.
    Just a thought.....
     
  15. blazedogs
    Joined: Sep 22, 2014
    Posts: 445

    blazedogs
    Member

    Thank you very much all Will work on it this afternoon again & check on your suggestions. Then will remove the booster. Can one bench check the booster off the car ?
     
  16. With a vacuum pump and gauge you can. Just use a hand pump vacuum, no gauge really needed. If it holds then it is good. Check the check valve and blow through it, then suck back. If you cannot get any air when you suck it is good.
     

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