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Brake booster type
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holden05 Offline
#1 Posted : Sunday, 18 October 2026 6:36:56 PM(UTC)
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Hi guys
Trying to work out the type of brake booster on a vc Brock. Is it a single or dual diaphragm
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Smitty2 Offline
#2 Posted : Monday, 19 October 2026 1:36:26 PM(UTC)
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Originally Posted by: holden05 Go to Quoted Post
Hi guys
Trying to work out the type of brake booster on a vc Brock. Is it a single or dual diaphragm


VC Brocks used the standard GMH 4 wheel disk brake system which had tandem (dual) diaphragm booster

HTH

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holden05 Offline
#3 Posted : Thursday, 22 October 2026 3:56:54 PM(UTC)
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Copy. The brakes are doing my head in
I’ve bled what I reckon is all the air out of the master, pretty much a bottle per corner.
The pedal feel i get is good, not soft, good travel
Thinking it may of been the booster, I’ve done the test where I turn the car off, pump up the pedal till it’s firm then turn car on. Pedal sinks about 5cm
Issue I’m having is the feedback is crap, I’m having to bury the foot to stop and the brakes feel really “wooden
Is this as good as it gets or have I a problem?
Car has been in storage for a while so would it be pads?
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HK1837 Offline
#4 Posted : Friday, 23 October 2026 11:59:42 AM(UTC)
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Did you remove the master cylinder? Sometimes with these the reaction pad falls off and into the booster, giving you a terrible pedal once the master is re-fitted.
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holden05 Offline
#5 Posted : Friday, 23 October 2026 12:36:14 PM(UTC)
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Yes I did to get resleeved
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HK1837 Offline
#6 Posted : Friday, 23 October 2026 12:38:39 PM(UTC)
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This might be your problem. I have had this issue with a later UC in the past, same cylinder and style of booster.
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Smitty2 on 23/10/2026(UTC)
holden05 Offline
#7 Posted : Friday, 23 October 2026 7:10:31 PM(UTC)
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The pedal feels good, that’s the confusing part
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LAZZA67 Offline
#8 Posted : Monday, 26 October 2026 4:44:08 PM(UTC)
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Hi holden05,

This sounds like a process of elimination, and I would be starting at the brake calipers first mate, if you haven't already done so. You mentioned that your vehicle's been in storage for a while, so this leads me to think that either the caliper slides or pistons may have seized which is quite common on these vehicles. You'd still be able to bleed the brakes and get a great pedal, but if the callipers have seized, then they won't allow the brake pads to grip the disk rotor with any force.

Also, with the brake booster reaction disc, yes, it may have fallen out if you've overhauled the brake booster, but looking at the Holden repair manual, this disc is located behind the booster push rod and return spring, so I highly doubt it would have fallen out if you've only removed the brake master cylinder. From my experience, a missing reaction disc with give you a very sensitive brake pedal meaning your brakes will lock up as soon as you just touch the brake pedal.

Let me know if this is helpful.

Cheers
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holden05 Offline
#9 Posted : Monday, 26 October 2026 5:19:02 PM(UTC)
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Hey Lazza, thanks for that info, the calipers and pads are the only thing I haven’t touched. Best way I can describe it is pedal feel is good but brakes don’t seem to bite
Sure they slow the car down but I don’t know what it would be like in an emergency
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LAZZA67 Offline
#10 Posted : Monday, 26 October 2026 6:36:30 PM(UTC)
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Hi holnen05,

Definitely sounds as if the caliper slides or pistons are seized. If it's the slides, then potentially, only one of the brake pads on each caliper will be doing all the work. Might pay to inspect each brake caliper to make sure that both slides and the piston are free.

Cheers
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