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Hydroboost on clutch line (car)

  1. Jan 20, 2016 #1
    I want to install a hydroboost (VH44) to make the clutch easier to use (because of an injury/disability).

    From what I've read, it'll reduce the pedal travel distance *and* tension.

    What would happen if I installed two hydroboosters, in series?

    Reduce travel? Tension? Both? Nothing at all?

  2. jcsd
  3. Jan 20, 2016 #2
    Sorry, I meant vacuum assist - not hydroboost, which is ran by the PS pump. (no edit button!?)
  4. Jan 20, 2016 #3
    I think the distance of travel is typically governed by the bore of the master cylinder attached to it, the pedal pressure will be reduced for a given force on the master with a vacuum (or hydraulic) booster.

    We have an old Toyota Land Cruiser 6cyl diesel from about 1985 that has a booster for the clutch. it's a smaller diameter than a brake booster (about 4-5") and would be easier to find the room to mount.. Might want to look into that.

    I think booster units usually have a spring inside them to require a certain amount of pressure.. if you weakened the spring you could probably get an even lighter pedal, it may affect the sensitivity of it though.
  5. Jan 20, 2016 #4
    any idea why this guy ended up with less travel?... the other person in this thread also said that shouldn't have happened.

  6. Jan 20, 2016 #5
    I'm not sure why he ended up with less travel, unless he used a bigger bore master that bottomed out the pressure plate

    With a lot of time and parts, I think you could find something that works... you may have to play with pushrod lengths on the master-booster, and booster-pedal to get what it seems like you want.. lower disengaged height, and easier pedal.. The lower disengaged height will mean you need a bigger bore master cylinder to move the same volume in a shorter distance, the lower pedal pressure you're looking for will come from the booster. As long as the booster is capable of supplying the force needed to the master, you should be able to fine tune the pedal pressure by playing with the spring in the booster.

    The booster is nothing more than a vacuum reservoir, a diaphragm and a valve to control it.. As you press on the pedal, you compress a spring until it opens up a port allowing vacuum to help you.. weaker spring = less force to open the port and less pedal pressure
  7. Jan 25, 2016 #6
    The booster has a 5/8" bore.

    My master cylinder has a 3/4" bore.

    This means the pedal travel distance will increase?...
  8. Jan 25, 2016 #7
    I dont' think the bore of the booster is terribly important, I think it's just a slide valve.. you'd compare the bore sizes of the old master to the new master... Since the old master was unboosted, they probably used a smaller bore to increase the pressure, while the boosted ones use a big bore to be able to afford the same volume displacement with less pedal travel. I think there are some 7/8th and 13/16th ones (either was available on the Land Cruiser I think)

    You may also be able to fine tune it by changing the slave cylinder.
  9. Jan 25, 2016 #8

    I'm not replacing the master, this booster goes inline between my existing master and slave.
  10. Jan 25, 2016 #9
    If you're only adding a booster, travel shouldn't change..
  11. Jan 25, 2016 #10
    So, what does the bore size of the booster actually do?
  12. Jan 25, 2016 #11
    From the ones I've taken apart, the bore is just the valving system, similar to a hydraulic spool valve.. it's just the controller for the flow of vacuum from one side of the booster to the other and doesn't contribute much to the force either way (the spring behind it would to an extent affect the pedal force it takes to open the valve)
  13. Jan 25, 2016 #12
    Alright, ordered the booster. Thanks!
  14. Jan 26, 2016 #13


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    For those that haven't read the link in post #4, it says the booster "...has a small piston pushing a larger piston." Hence the decrease in pedal travel.

    The mechanism for the lower pedal force is also described.
  15. Feb 3, 2016 #14


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    What's preventing you from purchasing a vehicle with an automatic transmission?
  16. Feb 12, 2016 #15
    Autos Suck.

    Anyway, got my booster installed!!

    Pedal travel is Definitely reduced, when vacuum is applied. Without vacuum, it requires normal travel.
  17. Feb 12, 2016 #16
    How does it work for you? do you like it? I guess it'll take a little getting used to?
  18. Feb 12, 2016 #17
    I like it, but it doesn't work right. The pedal won't return fully, sometimes. It sticks where the clutch is worn, so it tells me I need a new clutch. But more importantly, I don't understand why it sticks at all and doesn't return all the way... why does it need my assistance? why isn't the existing tension, though low, sufficient?

    Also, sometimes the clutch will not FULLY ENGAGE (I do not mean disengage). This is my biggest concern. The most recent time, I was able to turn off my car and pump the clutch pedal with limited pedal travel and get back on the road, and it worked right. But why did it happen at all? also, it happened after shifting @ 4k.
  19. Feb 12, 2016 #18
    best I can suggest is a helper return spring to make sure the booster doesn't need to push the pedal back... I know it was mentioned earlier but don't know of you have it installed
  20. Feb 12, 2016 #19
    nope, it still has the overcenter spring though - that should help it return?
    if I add more return, what was the point of adding the booster to reduce tension?... lol...
  21. Feb 12, 2016 #20
    is it sticking after the overcenter spring cams over? Perhaps try removing the overcenter spring first, and see how that affects it, you wouldn't need a huge spring to help it back, as long as the pedal comes back on it's own.
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