Lost powerbrakes after WOT?

Discussion in 'The whoa and the sway.' started by srb, May 27, 2021.

  1. srb

    srb Well-Known Member

    Is it normal for these cars that you have no power brakes right after WOT? It came like this when I bought it and I've replaced this connection once in the past. I suppose it's a check-valve, but that didn't change anything:
    I know there is no vacuum at WOT, so in that regard I figured it was just an old car feature. And a second after the power returns.

    But, none of my modern car have this phenomenon, so perhaps it's not normal?
  2. Briz

    Briz Founders Club Member

    I believe most new cars use Hydroboost.. The wifes Hybrid is all electric power acc.
  3. DaWildcat

    DaWildcat Platinum Level Contributor

    That's an odd one, Stefan.

    Even my over-cammed engine will make great vacuum at the brake booster once throttle is closed and rpm's are decreasing.

    Is power brake function ok during casual street driving?

  4. LARRY70GS

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

    The check valve is supposed to prevent that, so either your check valve is defective, or the booster is leaking.
    chrisg, 1968_GS400 and Max Damage like this.
  5. srb

    srb Well-Known Member

    As soon as the throttle is closed vacuum returns. Which gives an odd brake response, because it is very progressive. At first not a lot is happening and then full brake power is enforced with the same pedal pressure.
    During normal street driving everything functions well. It's only after wide open throttle, so basically sprinting away from the traffic light does mean no other car should accidentally try to cross. (That's more often the case, but in my case a little more).
    I don't think I have a leak either, because when I unplug the booster later when then engine is off there still is vacuum present.

    I don't know, but apparently it's not as it's supposed to be?

    Edit: ok. I'll fix the brakes when it's driveable again :D
  6. LARRY70GS

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

    Wait a minute, you are complaining about an issue and the car isn't driveable?
  7. srb

    srb Well-Known Member

    Haha, I'm currently restoring it. So yes kinda.
    I 'd like to know if I have to do something about it or not. If it were normal I wouldn't bother fixing it. Since it's not normal I have another task on my list.
    I expect the car back from the painter anytime soon, so then I can reassemble it again, also the brakes. So I'm planning my actions and thinking ahead and this was something that was nagging.
  8. LARRY70GS

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

    The check valve is defective or the booster is leaking.
    72gs4spd and Smartin like this.
  9. Prairie Piston

    Prairie Piston Well-Known Member

    I blew one of those check valves out heading into a 110 degree turn in the Riv. Had to stand on the brakes and arm wrestle the steering and then managed to get the car into a hotel parking lot to avoid a 4 way stop that I would have blown thru. Scared the {%[*] out of me. Still can’t believe I didn’t hit anything
  10. Jim27

    Jim27 Member

  11. srb

    srb Well-Known Member

    No is a vacuum hose. Don't know the size, but it looks correct.
    I'll order a new valve and check for vacuum leaks.
  12. stump puller455

    stump puller455 1970 GS 455

    in the last 4 months my gs has ejected the lid- cap on the booster (check valve thingy ) on start up.. when I back the key off to fast... upon stat up or a quick bump start
    docgsx likes this.
  13. stump puller455

    stump puller455 1970 GS 455

    who make a quality replacement
  14. LARRY70GS

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

  15. docgsx

    docgsx It's not a GTX

    Same thing happened to me, after that I kept the spare in the glove box
    stump puller455 likes this.
  16. stump puller455

    stump puller455 1970 GS 455

    Well its more like a cough / or small backfire engine rotate backwards slightly.. if i hold key down and crank over engine it never happens.. seems only when i hit the key quick expecting a quick snappy start ...and good by booster vac check valve the middle disappears like a missle The booster is new as is the grommet engine runs great
    docgsx likes this.
  17. LARRY70GS

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

    That is weird. I don't think that has ever happened to me, in any car. There should never be ANY pressure inside the booster if the check valve actually works. Booster is new, since when does that matter?:D
  18. srb

    srb Well-Known Member

  19. srb

    srb Well-Known Member

    I have, but the hissing sound is somewhere from the rod. So that won't help.
  20. john.schaefer77

    john.schaefer77 Well-Known Member

    Have it rebuilt if it is an original. It will be better than a store bought mass unit. IMO

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