Baxi 105e Intstant - Autobleed valve Open all the time?

I have a Baxi 105e Instant combi. The heating is an underfloor system run from this boiler.
The pressure on the boiler seems to have dropped to below 1/2 bar. It has done this twice before, which led me to think it was a leak.
The underfloor heating circuits were tested for leaks during installation and were fine - No leaks.
The second time this happened i thought it might be the fact the auto bleed valve on the pump was left open. Is it ok to leave the auto bleed valve open?
Anyway it's closed and the pressure still dropped!
There is also a squeal type noise when the boiler kicks in (either from heating demand or hot tap). I think this may be the pump or diverter valve. Would running the system at low pressure cause damage to these devices?
There are no signs of leaks from the actual boiler itself, so i don't think it is the expansion vessel.
The only thing that is a bit suspect is the UFH manifold. I never checked this and there is (very) slight evidence of a leak around one of the ports. I can't see how a 1bar pressure drop could produce such little water?
Anyone have any advice (apart from calling a plumber!)
Regards
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On Mon, 09 Feb 2004 07:56:39 +0000, Reader wrote:

Quite possibly.

This will be dependant on the expansion vessel charge pressure (See FAQ). Once the expansion vessel diaphragm is against the water inlet the pressure will draop off very rapidly indeed.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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The fact it only squeals when the boiler kicks in, leads me to think it's thew diverter valve. Does this sound ligical? Is there anyway to test if it's the diverter valve? Stethoscope?

Thanks for your help.
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