Yes the three way valve can be cleaned out! I've been there on a very bad dog of a Discovery 300TDi (the white one) that gave me a LOT of trouble. Fortunately I had already bought hundreds of pounds-worth of spares for an even worse dog of a Discovery (the red one) that went belly-up after three days. (No fins left in its radiator and severe overheating on the Blackdown Hills on the A35 followed by more overheating on the A30) The sellers were out and out cheats as in the end it turned out that the father had bought the car for his 17 year old son and lent it to him for eight years! As the son's hobby was off-roading you can imagine what the car was like. Of course the legal owner was the middle aged father who purported that the car was his and had been fully serviced by a garage.
Now regarding that valve or three way union. Get a long thin wood-screw and keep screwing and pulling until all the hardened sludge comes out. On my white Disco the pipe stubs were completely choked with either rust or radiator sealer. If the non-return valve diaphragm gets damaged it doesn't matter as the late types are just are three way union. Finally connect the pipes and union to a water tap and make sure that water flows freely through ALL the pipes.
The white Disco had also overheated and blown its head gasket just like the red one but if I had known about the blocked union it would have run for a lot longer. In actual fact somebody (not me) had messed-up the valve timing and all the pistons had valve dents in their crowns. My repairs included new standard pistons in cylinders which had been ridge-reamed and honed by myself. A new South African cylinder head was fitted and a new aluminium radiator that had a 10% upgrade over standard. A new timing belt kit was fitted and it was TIMED CORRECTLY by yours truly. After all this work there was no more overheating and towing 3.5 tons was no trouble at all even in hilly districts. Some pundits claim that fitting new pistons in old bores is bad practice but after my repairs the white Disco hauled a 4 wheel boat trailer and its load from Peterhead to Cornwall. No water or oil were used apart from diesel oil and this is proof that a Disco can be reliable. Good luck!
PS A GOOD Disco is able to tow in 5th gear but a bad one like the red one will not accept 5th owing to excessive lash in the transmission (it will snatch and jerk if 5th gear is engaged when towing) Of course the problem when buying a car is that one seldom has a 3.5 tonne trailer and a range of steep hills available. These are IMHO essential when trying to differentiate between a Disco that can really pull and one that is only any good for the school run. In my case my twin-cam hatchback had snapped its cam-belt over 200 miles from home and it was then found that my recovery package had slyly been watered down to a tow of twenty miles maximum. I did have a policy booklet extolling the full recovery and/or hotel package that was included with my car insurance but this must have just been a first year introductory offer. As the RAC wanted about £500 to recover the twin cam, the red Disco was purchased but it turned out to be a dud. In the end I had to repair my Rover 800 Di (the beaten-up one that was hit by an uninsured artic) and recover the twin-cam using that. In my long experience no-one ever sells a good 4x4 and all the affordable ones are basically like a Meccano Set (you have to build them yourself) Good luck!
