Combination valve

Discussion in 'The whoa and the sway.' started by Mart, Aug 7, 2021.

  1. Mart

    Mart Gold level member

    All you brake experts, what is the result if that little tool isn't used on the combo valve during bleeding?
    Do you end up with less front or rear applied pressure?
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No, but it may make bleeding more difficult. The hold off section holds off front brakes until a preset pressure is developed in the rear braking circuit.
  3. Mart

    Mart Gold level member

    Bleed seems fine. Both frt & rr fluid reservoirs on master went down equally during bleeding. Relatively new booster (&master) hisses when check valve is pulled.
    Vacuum storage canister just added. Not much change. Pulled rr drums today, broke glaze on shoes & pads, adjusted them up for light drag, no change.
    What size master cylinder piston bore is required for disc drum? 1"or 1-1/8"?
    I want my brakes to lockup if heavy pedal push.
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    1" would give you more hydraulic pressure, but I am not sure you will be able to get the brakes to lock, not all 4 wheels anyway. It is 50 year old braking technology. You may need to go aftermarket.
  5. Mart

    Mart Gold level member

    When this car was new(er) the factory 4 wheel power drums would throw you thru the windshield.
    I've never been able to get that braking response back.
    I think I have a 1-1/8" corvette disc/disc master on now. I bought but haven't tried yet a 1" bore disc/disc.
    The original power drum master was 1".
    This car runs way too good to not have great brakes.
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Yeah, I have never been really impressed with the braking power of my car. It definitely has more GO than WHOA, but it does haul me down from 116 MPH at the track pretty quickly.
  7. Max Damage

    Max Damage I'm Working on it!

    Drum brakes have more mechanical advantage for locking up the wheel, and old tires were inferior in friction to modern ones.

    My car with Disc/Drums and GM master/Booster, will definitely lock up the rears (drums) if you just stomp on the brake, but the discs don't seem to lock. They do stop the car very quickly and evenly...
  8. Mart

    Mart Gold level member

    Maybe something to be said for ceramic pads/shoes. I kind of came from the asbestos composition era.
  9. Mart

    Mart Gold level member

    I have a retired Cadillac mechanic buddy that says he needs to drive the car to make an assessment.
    He knows his chit. He did my alignment. He raced roundy rounds and has a 64 vette with 13,5 comp n/a. I'll wait for his assessment.:D
    Max Damage likes this.
  10. TrunkMonkey

    TrunkMonkey Well-Known Member

    The "tool" keeps the combo valve piston in the normal/centered position to allow the brakes to be bled.
    While bleeding and without the tool installed, the loss of pressure causes the piston to "lockout" (block) the front or rear brakes having the low pressure during the bleed. Once that occurs, you will only have pressure on the front or rear system that was not being bled.

    Example, no tool installed and you start to bleed the rears, the master cylinder builds pressure, both front and rear and the rear bleed screw is opened, and the pressure drops to the rear brakes, the valve piston then moves to block the rear to prevent further fluid loss and allows for front brakes to operate.

    If you continue to bleed the brakes, you will get nearly zero accomplished by bleeding the rears, and if you switch to the front, you may be able to get an adequate bleed on them.

    The only way to "reset" (most) compo valves is to open both front and rear lines, and press the pedal to determine if front or rear are flowing more fluid. Once you have that establish, close off the high flow bleeders and press the pedal with the low flow bleeders open and the valve should move back to the center position.

    The tool is the right way to bleed the brakes using the pedal method. (I use Russel Speed Bleeders. One way check valve bleeders that allow me to bleed my brakes alone.)

    And as far as braking goes.

    Just like "Spinning ain't winning", "Skidding ain't stopping." You want max friction on the contact patch, just before the traction is lost to "locked up" wheels.

    I worked military aircraft and anti-skid systems before cars had them. In the beginning they "pulsed" (using wheel speed sensors and accelerometers) and later systems with more computing power and sampling rates.

    The early systems you could see "tire patch stipes" sort of like dashes down the runway as the brakes would skid, then roll, then skid, rapidly. Occasionally, tires would flat spot or fail if the anti-skid was out of adjustment.

    Aircraft tires on landing fail within a second or two, and before an aircrew can tell it is skidding.
    The other issue is rubber reversion, when the tire skids and creates a layer of molten rubber and the tire effectively is the same as hydroplaning.

    Skidding tires on cars are no longer stopping and the secondary loss is of directional control, if the fronts are locked, and the car ends up in a "going sideways" you can turn the wheel, but Sir Isaac Newton is driving.

    If the backs lock up, you may be able to steer into the skid and have some control, but having max stopping without skid is what you want.

    Better to pay a good and reputable brake shop to ensure they are right, if you do not know what you are doing.

    A man's gotta know his limitations. Even if the punk feels lucky. Right, Clyde?
    Waterboy likes this.
  11. Mart

    Mart Gold level member

    Does the tool need to be used if bleeding using the no pedal method and vacuum method bleed?
    I also left the distribution block on frame undisturbed.
    knucklebusted likes this.
  12. knucklebusted

    knucklebusted Well-Known Member

    I do not think vacuum bleeding requires the tool since you are drawing it down and no pushing it with the pedal under pressure.

    If fluid is coming out of front and rear bleeders, you don't need to reset the combo valve.
    12lives and Mart like this.
  13. jaye

    jaye Well-Known Member

    I didn’t use the tool and I don’t have any pressure in either side of the fronts. I can get fluid but no pressure when pumping the brakes then opening the bleeder screw. Does this mean the combination valve is closed? No leaks
    knucklebusted likes this.
  14. knucklebusted

    knucklebusted Well-Known Member

    It might be. Try bleeding a rear brake and see if it clears up the front issue by re-centering the valve.
  15. Mart

    Mart Gold level member

    This youtube vid shows how you can check the position to see if the valve is centered.

  16. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The combination valve contains three sections,

    Valve centering refers to the warning switch portion. If the valve WASN'T centered, it would light the BRAKE light. That happens when you lose hydraulic pressure on one side, front or rear. Restoring pressure, after bleeding, and a hard pedal application will re center the valve.

    The hold off section is what you are concerned with when bleeding the brakes. The Manual tells you to hold in the valve stem when bleeding the front brakes. The valve stem is on one end under a rubber boot. You can use a c clamp to accomplish that. The hold off section "holds off" pressure to the front discs until the rear circuit develops a preset pressure. That is the reason for holding the stem in, so you get full pressure to the front when bleeding.
  17. Mart

    Mart Gold level member

    Ok, so if vacuum bleed method is used, are you getting the pre-set pressure to rears along with full pressure to fronts without touching that front stem?
  18. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No, you would have reduced pressure to the front brakes potentially making it harder to bleed them. That is why the manual tells you to hold in the stem.
  19. Mart

    Mart Gold level member

    I'm confused.:rolleyes:
    That above info says bleed left frt 1st...opposite of what I've always done my whole life.o_O
    I give up.....:D
  20. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Get the gf to help you bleed the brakes the conventional way. Forget the vacuum. Think she'll go for that? You don't have to run the engine.:D

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