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VH SLX, Would a V8 master cylinder fit on a 6 cylinder braking system

Discussion in 'VB - VK Holden Commodore (1978 - 1985)' started by griezy, Mar 6, 2020.

  1. griezy

    griezy Member

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    im having problems in my SLX VH wagon two years ago the master cylinder completely failed (brake pedal went straight to the floor, so i took it in to the mechanics (they installed a new master cylinder completely bleed the whole system and added new fluid),about a year later the pedal went strait to the floor again, took it back, and the mechanics put new seals in the master cylinder and bleed them again and on the 1/3/20 coming from near bathurst to campbelltown they failed again (3 hour drive)
    My question is would it be worth getting a new V8 master cylinder to replace the 6 cylinder one, can it fit without changing anything else or do you need a V8 booster as well?

    thanks in advance
     
  2. markovr

    markovr Well-Known Member

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    It will fit fine with no modification and should improve pedal travel....however it’s strange that the new cylinder needed new seals...a brand new one should last for years .
    A dud Booster can give similar issues with a long pedal
    .
     
  3. Vin999

    Vin999 Active Member

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    With boosters its more about size (thickness as single or double diaphram) and the length to the strut m/cylinder bracket bolts, rather than 6 or v8 option as vb-vh 6 cyl sle’s had standard d/d boosters the same as v8 optioned cars as well as cars that had factory 4 wheel disks, auto and air con fitted together.

    Check yours and then Upgrade your existing 15/16” m/c to the VL Turbo car 1” m/cylinder is a direct fit to your existing bolt pattern booster.

    Later cars the bolt pattern is different to your vh booster n m/cylinder
     
  4. losh1971

    losh1971 Well-Known Member

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    Wouldn't a 1" master require a different booster? 1" master places less force on the caliper pistons than a 15/16, unless of course it has some other internal design that increases the pressure. But from my understanding they fitted the larger bore MCs with a stronger booster to compensate.
     
  5. griezy

    griezy Member

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    Thanks guys hmm getting conflicting info first message saying it should fit with no modifications, last message saying I would need the booster as well,

    i had a look at the VL turbo - V8 master cylinder online but the front brake line is in the completely different spot (Vh front line is on the side of the master cylinder and the VL turbo / V8 one goes up from the bottom, would that means I would have to find a VL front brake line some where
     
  6. vc commodore

    vc commodore Well-Known Member

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    From VB upto and including VR have identical M/C and boosters....Just make sure you have the correct booster, as to what is on your VH....Reason being, where the bracket comes from the strut tower to the master cylinder is in a slightly different position, depending on booster you have....Saves modifying the bracket
     
  7. vc commodore

    vc commodore Well-Known Member

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    Na mate.....The 15/16 and 1" master cylinder use the same booster......Be it double diaphram or single diaphram....That being said, I personally prefer the double diaphram booster...

    Originallly, I had a 15/16 master on my VC, with a double diaphram booster.....I have since upgraded to an 1" bore M/C
     
  8. Skylarking

    Skylarking Well-Known Member

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    My understanding has always been that the pedal arrangement, via leverage, provided force to the booster which then uses vacuum to add to the drivers pedal effort. That way the driver feels less effort was needed. But the overall pedal leverage is fixed so that even with a failed booster, the driver should still be able to stop. The drivers effort and the amount of boost is what dictates brake line pressure.

    The master cylinder bore size is dimensioned to ensure the correct volume of fluid could be sent to the calipers to cater for pad movement while braking. Increasing master cylinder diameter means the caliper piston mover further for the same pedal stroke while reducing the master cylinder diameter means the caliper piston moves less for the same pedal stroke. Thus, you can get a leverage change based on changing the master cylinder diameter but I’ve always understood leverage is mostly from the pedal arrangement.

    So swapping parts can have a play on other parts and blindly increasing master cylinder bore diameter would mean longer brake pedal stroke and what probably feels like less brake efficiency, unless something else is changed (like pedal leverage).

    That’s been my understanding and I’d be interested if anyone has references to and books if my understanding is wrong.
     
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  9. losh1971

    losh1971 Well-Known Member

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    I have experimented with different size MCs and the smaller bore definitely places more pressure on the caliper pistons and pulls you up quicker. The smaller bore also flexs the lines more. The larger bore generally is used on twin piston or more piston calipers. A large bore MC on single piston calipers and standard booster and all else being the same will feel like a rock.
     
  10. vc commodore

    vc commodore Well-Known Member

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    Speaking just about the Commodores, upto about the VZ, police cars came standard with 1" bore masters as standard....That's the way they wanted them. I know of a VP V6 ex-cop car, with a 1" bore m/c and pretty sure it only had the single diaphram booster....But don't quote me over the booster, as it was 6 months ago I had a quick look at it.....But I could find out...

    Just going by VB to VK,(I think upto the VK) the V8's were equipped with double diaphram boosters with either m/c fitted and the 6's the single, with the 15/16 m/c....
     
  11. vc commodore

    vc commodore Well-Known Member

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    It is strange you mention this.....I had a booster fail on me and it actually caused my brakes to lock after about 20 minutes of driving....

    I will try and find the thread I posted up about this issue about 12 months ago.

    Found the thread

    https://forums.justcommodores.com.au/threads/brake-issue.276451/#post-3023207
     
  12. Fu Manchu

    Fu Manchu Well-Known Member

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    I went to a 1” master on my VP ute years ago and put a V8 booster on.
    I recall I had to change the pipes to suit a different flare design and a different bracket for the master to strut Tower.

    I don’t recall what I did with the proportioning part.

    Why not just change the booster too? It’s no biggie.
     
  13. vc commodore

    vc commodore Well-Known Member

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    I've had all combinations on my VC...single diaphram booster, with both M/C's and double diaphram booster with both M/C's....

    The only difference is the bolt positioning on the strut tower.....The bracket is identical, however if you change the booster from one to the other, from what was originally fitted, you'd have to modify the bracket to suit :)
     
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  14. markovr

    markovr Well-Known Member

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    I think your problem was caused by the booster having too much pushrod length and after a few applications it was not allowing the cups in the master cylinder to come back far enough to expose ports for the fluid to return to the reservoir...so it sort of like driving with your foot on the brake pedal.
    I have found when a booster fails it can cause excessive travel but also then it can also make the brakes slam on right at the floor depending on wear dimensions and tolerances but the problem wont disappear when you try to bleed the master cylinder like you have.
     
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  15. Vin999

    Vin999 Active Member

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    Whats a large bore ???? you experimented with ???

    You might be getting too confused Losh, the large bore was used on single c/piston that used a larger disk. On a twin caliper pistons and more, an even larger bore m/c with quickfill benefit to cater for more pots and bigger disk/pad sizes was used.
     
  16. losh1971

    losh1971 Well-Known Member

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    I have experimented on the RD but essentially its the same principle.
     
  17. Vin999

    Vin999 Active Member

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    Get the vb-vk 1" then that was fitted to v8 dealer police and hdt cars and your lines will match, and as a few of us have mentioned depending on your booster size you might need to mod the strut m/c bracket. Easier to take old m/c in and match with new one in shop, if you don't want to research/understand all the different booster and m/c options.
     
  18. Vin999

    Vin999 Active Member

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    having just been to the pub ........ whats RD ?
     
  19. losh1971

    losh1971 Well-Known Member

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  20. Lex

    Lex Well-Known Member

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