FE-FC Holden Discussion Forum

I don't think my newly fitted second hand VH40 is performing up to scratch and suspect it may need a new diaphram.  Can anybody tell me if the booster should suck enough air through it to affect the fuel mix and should it suck you hand to its breather below the cylinder/body if you put your hand over it (when the pedal is depressed)?  

It is my reasoning that if it is creating a vacuum then air should not actually be passing through it and if air is passing through then the diaphram is rooted, am I correct?

Hi Burnsy,
Yep, the diaphram's rooted.
The actuating body is on the atmospheric side so shouldn't have vacuum there. The booster shouldn't affect manifold vacuum. Easy way to check it is run the engine and pull the hose off the one way valve in the booster, you should hear a pok sound of the diaphram snapping back into position and the engine will run rough. Replaced mine recently, cost me $24 for a new one from Better Brakes.
cheers,
Leigh

How involved is the diaphram replacement, could I do it without a manual, simple pull it out and replace?

Diaphram replacement is fairly easy as you don't need to mess with the hydraulic side. You should be able to replace it in position as all you do is undo the rear mount bracket & move out the way, release the spring band around the unit and pull the back away. You need to ease the small vacuum hose from the actuating unit while doing this too. The diaphram mounts on the centre of a big tin pad by means of a disc/nut set in the middle of the pushrod end. A little rubber grease or smear of brake fluid helps the new diaphragm fit back over this but it's real easy going.
An extra hand is a help when reassembling as you need to hold the halves together whilst doing up the band again. The diaphram seals against both sides, so it needs to be held central.
cheers,
Leigh

I have a spare unperforated VH40 diaphram that I'm selling (was bought for my booster, but it needed a full reco instead and they replaced the diaphram then, so now I have two and have sold the car as well!  Anyway $10 + postage will get it.

Peter

Just found I had the same problem as you and exchanged the whole unit, $180 well spent because it now has better stopping power than my 4 wheel disk landcruiser - have to fit my seatbelts before I send myself through the windscreen!!