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Vacuum Brakes


Automatic Isolating Valve Overhaul

by John Joyce

Page made possible by Dean Forest DMU Group, who have put together an overhaul kit to do this job. Thanks to them for supplying a kit for review and trial.

AIV overhaul

Here the valve is removed from the vehicle for overhaul. The attached gauge is not a standard fitment! There is a blanking plug which can be removed for a gauge to be attached in order to set it up correctly. Amazingly 79018 still had vacuum in the tanks, many years after they were last charged. Tempting then to assume that the seals are perfect!

AIV overhaul

With the bottom cover removed (two bolts). This spring acts against a large rubber diaphragm. The latter is open to atmosphere on one side and to the vacuum chamber on the other. When the force resulting from the pressure difference is high enough (ie the vacuum chamber is at >19"Hg) the valve is held open, enabling rapid brake release. The valve can also open of its own accord if the release pipe vacuum is higher than that already in the chamber, enabling the chamber to be evacuated in the first place.

Count the number of threads protruding before dismantling the unit so that it should be set up approximately correctly on reassembly. Then remove the two nuts carefully to avoid firing them across the workshop! Retrieve the spring and plate, and then undo the four bolts holding the two castings together.

AIV overhaul

With the lower casting removed. The diaphragm can now be seen. The long rod is the one that had the spring around it a minute ago. Lift off the diaphragm assembly.

AIV overhaul

With the diaphragm removed. The valve itself can be seen now.

AIV overhaul

The diaphragm assembly. The diaphragm is easy to change, but make sure that the holes in it will line up with the holes in the casting when you put it back together. The four holes aren't evenly spaced to it has to be the right way round!

AIV overhaul

Shows how the diaphragm assembly opens the valve

AIV overhaul

The top of the valve. Undo the hex-headed plug in the top to retrieve the valve itself. You may need a lot of leverage to shift it!

AIV overhaul

The valve assembly: hex-headed plug, spring and valve itself.

AIV overhaul

Undo the grub screw shown here to enable the valve to be dismantled and the rubber seal to be extracted.

AIV overhaul

Being refitted to the vehicle. Note that it can be a fiddle to get the valve plus new gaskets into place as the old gaskets were probably well squashed. It might be necessary to slacken off some pipe clips or whatever to enable the thing to be manoeuvered into position. How easy this is depends on the type of unit! One flange and gasket was bolted up quite easily; the second gasket was then fitted around one bolt and swivelled around that bolt into its correct position.

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