Thanks for the input Roy. Couple of questions if you don't mind?
How long should I be aiming for for the system to maintain a vacuum (if I don't use the lights when engine off)?
The system was designed with a vaccum storage tank and a one way valve, so that the vacuum created in the manifold when the engine is running creates a vacuum in the tank, and providing the system is air tight, it should retain that vacuum for some days without the engine running. So the first thing is that the one way valve must be in good condition to retain the vacuum side air tight. I have seen or known of some one way valves that are not sealing and therefore once the engine is not running, the air will simply come back in from the inlet manifold. In one case, a piece of debris had got stuck in the valve and prevented it from closing. So first check that the one way valve will open under suction, but close as soon as the suction stops.
As long as the one way valve is good, then the next most likely places for vacuum leaks are the rubber joins in the pipework, and the white shuttle valve at the front. The rubber joints in the pipework get hard with age, and crack or split, but you can replace these joints with new soft, tight fitting rubber hoses. The white shuttle valve at the front that controls which side of the double acting servo the vacuum is directed to, is known to leak after some years either because dirt gets in or the valves become gummed up and won't move or seal easily. Usually I have found a strip and clean should cure these leaking valves, but I know some have had difficulties getting them to work correctly, but now there is another option as a Murena owner has designed a more modern system, which is available through Matra Magic if you don't wish to do it yourself.
Finally, the double acting servo under the front can leak after years of use and normal wear and tear. It may have a split in the diaphragm or it could be the seal around the lift/lower activating rod. This should also tell you that the seal for the double acting rod should be a service point that you check and clean occasionally and lubricate to prevent that seal wearing!
Would it be prudent to wire it to the brake light test warning button?
Thanks in advance
Terry
Strictly you don't wire it to the test button, you wire a sensor to the warning system, and the test button is simply part of that system. If you are fitting the vacuum sensor in the front compartment, then you already have a suitable wire to the brake fluid cap, so simply link it there, or use the now unused left front pad warning wire.
If anyone has a Murena with the low vacuum warning light in the instrument panel, which is the early 1.6 cars (and possibly some of the very first 1981 2.2 models, although I'm not sure if they ever had one?) then all those cars need is a low vacuum sensor inserting into the vacuum sysem and connecting to the provided orange warning dash light, to use that already fitted system; but if you have a 2.2 or later 1.6 where that light has been used for the choke warning light instead, then the ideal is to use the red warning light provided for the low brake fluid and low pad material warning, which saves having to fit any additional wiring and an additional warning light somewhere else.
Now, the low brake fluid/pad warning light is also provided with a test button which is to check the bulb is still working, as normally you might never see that bulb lit if the car is used frequently and the system does not leak, certainly not often anyway if the routine maintenance is done and you note visually when the fluid is getting low, alerting you to check for pads nearly worn out, or a potential fluid leak. However, in the case of a vacuum sensor linked to a warning light, it is probable that the light will illuminate when you first turn the ignition on, if the vacuum has dropped, so that in itself will check the bulb is working. As you say, when you start your engine, the light comes on, then flickers and goes out, as the vaccum builds.
So if you connect the low vacuum sensor to the low fluid system, (the black & red wiring is for this system) you can use the test button to check the bulb any time as normal, but if you use the provided low vacuum warning light already fitted in the early cars, then there is a simple way to test if that bulb works, assuming the system is air tight and still has vacuum, so the warning light is out when the ignition is turned on. Simply operate the head lights a couple of times without the engine running to deplete the vacuum in the system!
Roy
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