Grohe Rapido-T Plumbing Assistance Required!

Hi
Before I go mad I'd like some suggestions on how best to fix my weeping shower couplings.
Some background: I'm in the process of completely re-fitting my en-suite, and I'm installing a Grohe Rapido-T concealed shower mixer (35500) which will feed a fixed rainshower head and a hand shower. Before I board and tile out the wall I wanted to pressure test my plumbing to make sure it is OK (no going back afterwards).
The problem is that, no matter how hard I tighten up the 1/2" to 15mm coupler feeding the outlets one of them still weeps when I force the outlet to be closed. I know in practice that the outlets will always be open so there will be no pressure on this connection, but I don't think that is the point (unless I am wrong?).
The weeping is coming from under the coupler where it touches the mixer socket. The socket body of the mixer is so deep as to mean the bottom face of the coupler doesn't come up against any face so I can't use a fiber washer. I've tried plenty of ptfe to no avail.
One thing grohe have used is a rubber ring on the sealing plug which I have removed to fit the coupler. Is this the way to go? If it is I can't see anything suitable in either the screwfix or toolstation catalogues.
Any other pointers before I pay somebody with more experience than I to fix my problem?
Shower photos: https://picasaweb.google.com/kev.holohan/ShowerPlumbing?authuser=0&feat=directlink
(http://bit.ly/IdKQvz if the above wrap is broken)
Coupling I'm using: http://www.screwfix.com/p/male-coupler-15mm-x/69358?_requestid †095
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I would put a fibre/poly/rubber washer on the thread before screwing it in. They have a parallel thread so aren't designed for PTFE tape seals.
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dennis@home wrote:

PTFE is designed for parallel threads, of course. .
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No it isn't.
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On 29/04/2012 14:57, dennis@home wrote:

PTFE works fine on parallel threads. It is very soft. Excess will extrude if you put too much on.
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parallel treads are not designed to seal at all. The fittings with parallel threads always have a shoulder which is where the seal is supposed to be made. Look at any parallel thread fitting like a tap connector. PTFE tape is designed to seal taper threads and is a replacement for boss white and string. If you put enough packing in you *may* get a seal but it is not guaranteed and it will probably leak later.
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On 29/04/2012 11:50, Kevin Holohan wrote:

https://picasaweb.google.com/kev.holohan/ShowerPlumbing?authuser=0&feat=directlink
How about starting with a couple of these http://www.screwfix.com/p/adjustable-radiator-valve-tail-compression/15531 and then using 15mm compression couplers to join up with the pipes?
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Roger
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On 29/04/2012 11:50, Kevin Holohan wrote:

https://picasaweb.google.com/kev.holohan/ShowerPlumbing?authuser=0&feat=directlink
To be honest I'm not keen on using PTFE tape for compression fittings. PTFE creeps so the force holding the olive in place will relax over time.
I would follow dennis's idea of using a fibre washer which partly expands when wet. I would also prefer to have a watertight seal as there will be some partial pressure when in use. I also see no reason why PTFE tape couldn't be used as an alternative.
Roger's suggestion would also work.
What do the instructions say?
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On 29/04/2012 15:28, Fredxx wrote:

Sadly the instructions say nothing for how to connect to the outlets (or inlets).
I might phone Grohe in the morning and see what they say.
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On 29/04/2012 11:50, Kevin Holohan wrote:

https://picasaweb.google.com/kev.holohan/ShowerPlumbing?authuser=0&feat=directlink
I assume its this one:
http://cdn08.grohe.com/lib/1/tpi/1375383.pdf
It looks like the main difficulty you will have is getting a parallel threaded coupler to seal in a parallel socket. Usually these will have a fibre or rubber O ring at that makes the final seal between the two flat surfaces (the top of the socket, and the flange on the coupling) when done up tight. If you have a rubber O ring hat came on the sealing plug, then use that round the coupler.
You can get a seal on parallel threads with PTFE, but it takes a surprisingly large amount of it.
Alternatively, use a taper fitting such as:
http://www.bes.co.uk/products/140.asp#6685
That will get a proper seal on the threads with PTFE (or Fernox LS-X)
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Cheers,

John.

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On 30/04/2012 02:20, John Rumm wrote:

I spoke to Grohe technical. They say I'm the first person to report this issue and to just use more tape.
I'll get some taper fittings just in case. Should I be using tape with that type of fitting (or non at all, or say the Fernox)?
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On 30/04/2012 21:03, Kevin Holohan wrote:

Yes you must use a sealing compound or tape with a taper fitting.
What I can't quite follow is why you can't take the rubber o ring off the blanking plug from your second photo, and stick that on the BSP threaded end of your coupler? That would then give you a proper seal.
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John.

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On 30/04/2012 21:30, John Rumm wrote:

Thanks. I'll go with the rubber o ring first but I want a backup if that fails...
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Thats the stage when my bullshit indicator would start clanging loudly!...
So your the only one eh?, doubt it somehow..

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On 01/05/2012 09:01, tony sayer wrote:

Just shows how popular their product is!!
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HERE IS THE CORRECT PART. I bought the same shower fitting and read this th read. A little light rummaging on ebay however I came across DOWTY seals, f rom the hydraulic world. Cheap as chips and actually meant to do this job. They are a rubber insert inside a washer. You just tighten them up with mod erate force, and they never leak. PERFECT. I paid Β£2.59 for 10 1/2 BSP DOWTY seals, also known as bonded washers, delivered to my door. Hope this helps, and the shower mixer itself is utterly fabulous.
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On 5/26/2017 3:46 PM, snipped-for-privacy@pinesoft.co.uk wrote:

Dowty seals are brilliant, we used them all the time in my days in a research lab.
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