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We use Dowtys everyday at work.

I'd say that there isn't enough of a flat sealing face on the brake hose for the Dowty to work.

New copper washers will be fine .. always have worked... always will

I seem to remember making some special copper washers for the adaptor end, needed to be 11mm I.D and the standard 3/8" I.D for the other side of the Y piece. As Andrew pointed out the sealing area is narrow.

You need two different sized copper washers, as 1071 Bob pointed out. I found this out the hard way when I inadvertently fitted two identical larger washers when rebuilding my 850. I could not get the pressure switch and two way union to seal, and had trouble with brake bleeding. When I pulled it apart, the bottom washer had buckled like a belleville washer. As soon as I fitted a correct sized smaller washer my problem was resolved.

The two washers are detailed below.
The smaller washer goes below the Union while the larger washer goes above the Union on the banjo nut.