At idle & when pressing the brake pedal an instant engine "shudder" occurs. Revs don't drop but it almost feels like the engine is running on 6 or 7 cylinders. It is a factory 5.0L SL/E with 4 wheel discs, the engine has been rebuilt to standard spec but with a slightly different cam. The vacuum tree at the back of the manifold has only the brake booster and cruise control hoses coming off it. For testing I've blocked off the cruise hose & the shudder still occur when pressing the brake pedal. I can't hear a hissing sound when pressing the pedal in, but is it possible that the diaphragm in the booster is stuffed? Pulling the vacuum line off the booster has the same effect and causes the shudder too. Is it normal for these engines to start shaking when there is a vacuum leak? Any help much appreciated.

Thanks Mick, that solves it & explains the shudder. Will try and find a new booster.
cheers,
Rhys

The 4Wdisc booster is deeper (longer) and has the bracket from the strut to the m/cyl wrap around towards the front of the car - hiding the nuts fitted to the studs on the strut tower. It is sometimes called a "lumpy face" booster by those who only know VL & upwards.

Drum rear cars, and later basic 6s with 4Wdiscs (standard across the range from VN owards) have a shorter booster (smooth faced from VK onwards), where the bracket goes the other way - so you can see the nuts/studs - that's your clue to seeing what will fit. It's no good for your application.

VC-VH-VK-VL-VN-VP V8, and some VR that are factory V8, or an original V6 BT1 cop car, will have the same booster as yours, and it's a direct bolt-up. Check that bracket first - make sure it's folded around the same way as yours, that way you know it's the right length booster.

Late VR had oddball things going on with booster & master combos, so avoid them. VS have a plastic booster. VT onwards are totally different and don't bolt up without adaptation.

PBR part# VH512 (blah blah = datecode). stamped across top of the front 1/2 of the booster. this is a dual diaphragm booster disc/disc application.
as Mick says, VB to VR slx, sle, BT1, V8, SS, and I think HD towing package. Getting harder to find in good condition, usually rusted.

is this not what you currently have in the 5.0 SLE?

Cheers
Balfizar

quote:

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Originally posted by Rhys
Thanks Balfizar,
Ended up going for a rebuilt exchange PBR booster exactly the same as the one on it. Fitted it, problem seems to have mostly gone away but there is still a moment of running rough for the first few seconds after the brake pedal is pushed. I assume this is from the booster "taking up" available vacuum.

Any other ideas Mick? The carby was "rebuilt" before I got the car, is it possible that something could be amiss with it?

cheers
Rhys

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Rhys,
Some pretty simple checks:- idle neutral / park
1 disconnect and plug the manifold to booster hose plug @ the manifold (test brake pedal ) if you still have a rough idle after and corresponding to brake pedal application, you have an electrical problem (dodgy wiring or earth point in the stop lamp circuit and I mean totally wired incorrectly in voltage source point and possibly a dodgy earth.(really very little chance of this)
2 fit vacuum gauge to the manifold and watch for deflection on application of brakes.

given a new booster it is hard to believe it is vacuum related.

Cheers
Balfizar

Thanks guys,
It does it when both stationary and moving, but more noticeable when stationary. The first test I did last month was plug up the booster and cruise vac ports, and no change when pressing the brake pedal, so that eliminates any electrical issues. Have left the cruise blocked off until this problem is sorted.

I made sure the pad everything else went back as it came out, and the actuating pin was clean and free of rust which I've seen on other cars, so it moves freely in the hole in the piston.

The reco booster could've been sitting on the shelf of the brake shop for years, who knows... so good suggestion, I'll take it back and get them to test/replace if necessary.

One other thing I found strange was that the master cylinder was the 15/16 type - I was under the impression that all factory VH V8s would be fitted with a 1". However looking into it further, some had 15/16 and others 1". It might be worth taking the opportunity to upgrade it to a 1" if worthwhile?

Problem solved!
The first "reco" booster was a dud (perhaps a dulux rebuild?) and the problem has gone away after fitting the replacement one. Took the opportunity to fit a 1" master cylinder too.

thanks all for the help.