Sh1t Pit Inspection chamber in the house Replacement

Hi All, Job number 671 has reached the front of the queue. The main bathroom. It was gutted 7 or so years ago and is now on the hit list.
First slight problem is, in our downstairs hall-way (built on a hill so one side is underground) which is the bedrooms and main bathroom there is an inspection chamber "collector" fed from the upstairs dirty water, a clay pipe from one end of the downstairs bathroom and a poo pipe from the other side of the bathroom.
The inspection chamber "top" is a double lipped frame packed with grease into which the single lip of the heavy chamber lid nestles and is then fixed down by 3 (originally 4 but one is sheared off) large brass screws.
Having such a lovely beast in the hall between bedrooms I'd prefer to replace it with something more "modern" that doesn't rely on loads of grease to make a gas/excrement/fluid seal but still needs to be mechanically locked in place should the outlet become blocked and sh1t backs up flooding the hall.
Current steel lump is 600mm by about 740mm (whatever the imperial size would have been)
Any suggestions of should I just grease it back up and forget all about it.. ?
Cheers Pete
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jim wrote:

Am I alone in thinking that air, being thinner, would be less likely to leak through than water, or is it more complicated than that?
Chris
--
Chris J Dixon Nottingham UK
snipped-for-privacy@cdixon.me.uk
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Unlikely you are the only one likely to get that so wrong.

Yes it is. Most obviously with PET bottles of stuff like beer.
The CO2 will eventually diffuse thru the PET and the water never does.
And with plumbing, water and shit laden water etc can see that blocking small unpressurised leaks whereas air doesn't do that with leaks as much.
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I guess it depended if it was a private or Water Board Chamber.
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