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Technical Brake pressure question

Discussion in 'Traditional Hot Rods' started by AGELE55, Jan 19, 2021.

  1. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    I'm feeling a bit stoopid..
    I collapsed my LF brake hose which made for a real holy crap drive. Car swerved violently. Luckily I was only doing about 35mph.
    So I get it home and see the hose is bulged and wet. When I apply brakes, the wheel locks and stays locked. I just replaced the hose and the wheel rotates and I'm waiting for my buddy to swing by for a bleed. BUT... I jump in the car and pump the brakes, I have zero resistance on the pedal. Seeing as the rear was never touched, why would I not feel something from the rear brakes engaging? Prior to hose replacement, I spun the wheels and had pedal to stop them.
    Things that make you go hmmmmm?
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 50,195

    squirrel
    Member

    If it's a single master cylinder, then that is normal behavior. If it's a dual master cylinder, then the pedal setup is wrong, and the dual master cylinder is not doing it's job of trying to save your ass from brake failure. Might as well have a single cylinder on it.
     
    AGELE55, Budget36 and 2OLD2FAST like this.
  3. Budget36
    Joined: Nov 29, 2014
    Posts: 7,498

    Budget36
    Member

    You hopefully replaced all the rubber hoses and have lost fluid. Fill the MC up and bleed all the brakes when your buddy gets there.
     
    Boneyard51 likes this.
  4. Maybe look at your rear brakes; do they need adjustment? shoes worn out. Good front brakes can cover a lot of ills in the back. Learned from more than a few salt rusted brake lines on my pick-ups.
     

  5. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    It's a dual MC with 4 wheel disc. Replaced the bad hose and it bled well. Prior to bleeding, I had my buddy pump the pedal as I tried to rotate the rear wheels. Even though he felt no pedal resistance, I couldn't rotate the wheel by hand. Test drive felt pretty good. Stomping the brakes would nose dive us pretty good and no swerving. Maybe I'm over thinking it.... Or, maybe the proportioning valve is funky..?
     
  6. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,611

    2OLD2FAST
    Member
    from illinois

    Just a word about brake hose failure . I don't remember brake hose failure being a " thing" in the late 60's -70's , didn't seem to happen much . Then I thought about how there were very few 40's-50's cars still on the road then . Now days we see many 80's-2000's cars on the road & we see many brake hose failures . I think there's no difference in the hoses or the fluid , I think its more , older cars on the road . In 1970 , a 1955 model was an Old car. 15 years old , In 2021 , a 2000 model is 21 years old but not considered an old car..
     
    Last edited: Jan 20, 2021
  7. Torana68
    Joined: Jan 28, 2008
    Posts: 1,285

    Torana68
    Member
    from Australia

    the brake pedal should be hard and not move far, your description doesn't seem to match this. Got a photo of the master cylinder?
     
    Special Ed likes this.
  8. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    It’s an old cast iron vette MC. Lol...if the description matched the problem, I wouldn’t be questioning it...
     
  9. squirrel
    Joined: Sep 23, 2004
    Posts: 50,195

    squirrel
    Member

    If the vette master cylinder was installed to replace the original master cylinder (I'm assuming the car is the 55 in your avatar), then the pedal will not use the full piston stroke of the master cylinder, and the "dual" feature will not function properly. When one side of a dual system fails, the other side will still work...IF you can push the piston far enough. Since the original MC had a shorter stroke than the dual MC, this won't happen.

    Now you know
     
    winduptoy likes this.
  10. chopolds
    Joined: Oct 22, 2001
    Posts: 5,983

    chopolds
    Member
    from howell, nj
    1. Kustom Painters

    Also, depending on what type of rear disc you have, the calipers might have to be adjusted to compensate for disc wear. The early GM ones, like on the 80's Camaro, had a real problem with this issue. You actually had to screw in the pads until they almost contacted the rotor. Had something to do with the mechanical parking brake set up.
     
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 50,195

    squirrel
    Member

    When the calipers are new, or in good condition, they will adjust themselves, if needed, every time you use the parking brake. If you don't use the parking brake (perhaps it's not even connected), then you need to do some work to get the calipers adjusted.

    There are lots of things that can contribute to these kinds of problems. There's a simple test to see if the brake system is set up to deal with a failed line...simply open a bleeder, and see if you still have some pedal. Then close that bleeder, and open one at the other end of the car, and see if you have pedal. If not, your fancy modern dual master cylinder is useless.
     
  12. Boneyard51
    Joined: Dec 10, 2017
    Posts: 5,349

    Boneyard51
    Member

    The problem is the “pumping” the brakes. Never pump or “build a pedal” when bleeding brakes! Air in a brake system can do crazy things, number one crazy thing is shoot air all through a brake system when pressurized. The best way to bleed a system is gravity or a pressure bleeding system. If you have to bleed with the brake pedal , fill your master cylinder and got to the furthest wheel, usually right rear . Open the bleeder and have your buddy mash slowly the brake pedal down. Fluid will or should come out the bleeder. When the flow stops, close the bleeder. Have your buddy let the pedal up, wait a 10 seconds then repeat until you have a flow with no air. Fill your master cylinder. Do all four wheels.
    When on the line I would do brake jobs and never pump the brake pedal once and get perfect brakes. Also remember if you are not replacing the master cylinder and you push an old master cylinder all the way down into an area where the cup has not been in years, your master cylinder will probably go out shortly! Just my experiences.






    Bones
     

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