Short version...

I need a 12v DC valve that will hold back about 4 Bar of pressure, but
doesn't restrict the flow when water with a 1m head is flowing, aided only
by gravity...

Full Story....

As part of my irrigation system, I have recently purchased some more water
storage capacity in the shape of 5 1000L tanks
(like this
http://cgi.ebay.co.uk/ws/eBayISAPI.d...tem=7611463784)
(...and I didn't pay anywhere near that price either!)

Anyhow, I have put three of these at the top of my garden (they are hidden
behind the garage!) the other two, and the original 2 x 1500L barrels (with
the irrigation pump in) are down the side of the house.

The three behind the garage are about 1.2M higher than the ones by the
house.

Now, my master plan is to have various float switches in the bottom group
and top group (each group is interconnected), so when the bottom group is
say, 90% full, it pumps 10% (of the total capacity) up to the top, via the
irrigation pump and a solenoid valve (if the irrigation pump is turned on,
the water won't go anywhere until other valves are turned on) as long as the
upper float switch on the top group has not activated (When they are full!)
The main reason for doing this is, the majority of the water is coming from
the house roof, the garage roof is a lot smaller (this will be feeding into
the top tanks too, of course!)

Now that is the easy bit, in order to get the water back down again to where
the pump is (& without running more pipes) I will need two one way valves
and a electrically operated valve that doesn't restrict the flow too much
(as the water returning will be falling with the aid of gravity) I tried a
solenoid valve I am using for the sprinklers (One of these
http://www.betavalve.com/wras.asp) but with my very technical test of
blowing though it when it was open, I can't see it working, especially when
the water level in the top tanks is low.

The plan is...

On the higher tanks, I will have a pipe to the main irrigation delivery pipe
(25mm MDPE), with a one way valve inline, so water can only exit at this
point.
I will also have another pipe from the MDPE with a 12v solenoid valve for
the lower - higher transfer of water (restriction is not a problem, as this
will be delivering water via the pump)

On the lower tanks. I will have a pipe from the MDPE loop connected to this
new low pressure valve, so when the pump is off, and the valve is open, the
water will return to the lower tanks aided by gravity alone.

The pump will also have a one way valve, so water does not return from the
top tanks via the pump, when the pump is off.

All my valves are the 12v solenoid ones like this
http://www.betavalve.com/wras.asp and I would rather keep mains voltage away
from the system (ignoring the pump, as I don't think I am going to find a
low voltage pump of this capacity, and I already have a mains voltage one!)

Any ideas where I can find such a valve, (I want to try to avoid having
another pump up the top as the hole in the top of the tank is only about
15cms wide, and I don't want to cut a bigger hole, as this would hinder the
lid fitment!

Thanks for any suggestions!!

Sparks...