Old Leafsprung Land Rovers

Looking through my hoard of series three spares today.
I'm bound to have another series three at some point,  and that is my excuse for not selling them on.
Anyway I found one of those pressure switch things wot tell you if one of the dual brake circuits is loosing pressure.
It's in a really rough looking state.
Can I get a seal kit for it like you can for master cylinders and wheel cylinders
Or is it non-repairable

Buggered if I can remember which one but either LRM or CLR had a recent article about rebuilding the PDWA valve. I think they had to source the parts based on measurements of the seals as there wasn't a kit.

<edit>

http://siteground237.com/~gunsandr/showthread.php?t=1379 - a chap has done it here.

Stripped the thing down today without any issues.
Came as a bit of a surprise TBH because I'll come clean and admit that I actually DUG THE THING UP whilst digging my back garden
IIRC it came off a 1984 SWB diesel that I broke up around 2006.
A tiny little bit of corrosion at one end of the piston thingy but nothing to worry about.
With reference to the possibility of the seals failing and allowing your dual circuit brake system to effectively become single circuit,
I had the idea of drilling the casting in the "dry" area where the switch unit is located.
Then if either of the "O" rings failed,  brake fluid would simply **** out onto the chassis rail.
Fluid level in the reservoir would drop - low fluid level warning switch activated - problem revealed.

OK so it's a bit overkill on an old leafer, just me hyphothosising (**** me that's a big word for me,  especially as I've just got back from the pub)

I can't see how it can "shut off" anything TBH
Yes I can see that it would be possible to build such a device with the addition of a few more seals but this is just a warning switch

A Haynes manual is a very dangerous thing if you are stupid enough to believe anything written in there :****:
There's an easy way to put this to the test.
Back off a pipe union and see if the thing does indeed shut off the fluid flow.
Or if you like drama,  get someone to stamp on the pedal while you take an axe to the brake pipes

TBH I'm not even sure how a series three with dual circuit brakes is configured.
IIRC some cars have a diagonal split,  but I can't ever recall seeing dual pipes to the rear axle of a leafer, so I'm concluding that the split is front/rear.
It looks like the PDWA valve also doubles up as a union/splitter for the brake pipes.
So at one end you have a pipe from the m/cyl in and two pipes out for the front brakes, and the other end one in and one out for the rear brakes.
Certainly at the rear pipe end there's no way that it can cut off the flow.
There's simply a split nylon spacer thing around the moving piston.
It's not designed to seal anything,  fluid can pass through the gaps in it.
If the piston moves up the other end, it could possibly block the flow to the union in the centre,  but the "seal" would just be metal to metal - not very clever 

Never had one of these but is it not the case that the fluid reservoir has a baffle in it so that when one part of the system has a leak the other still has sufficient fluid to allow the master cylinder to pressurise the other part of the system.