leaking D screen

You most probably have an old seal which is rubber and its seen better days. Sounds like you need a new rubber seal.
Alternatively, if you cannot do it yourself go to a windscreen fitter and ask them if they can reseal the screen with the appropriate
sealant that allows a bit of flex. This should be a relatively easy job but it does depend on the condition of the seal.
 
It may not actually be the bottom seal that is leaking, water can run along the inside of the windscreen frame and drip down behind the dashboard. The grey seal on the top & sides might have gone hard and not be sealing anymore or the black rubber at the base of the screen may not be sealing. There's a few things you can try:

Loosen the 3 clamps at the base of the screen and stretch the black rubber to pull it back against the glass.
Loosen the clamps and lever against the steel band at the base of the screen to push it further into the grey seal
Pump mastic in behind the grey seal
Replace the black mastic at the base of the screen
Replace the screen seals.
 
There are two forces at play here. Gravity and wind pressure. The DS glass securing method is less than sophisticated relying on a good lip seal across the base. The screen is not well shaped to match the scuttle pressing ( or the other way around ) there is a huge amount of packing required to force a good contact upwards around the spaces behind the bonnet hinges. The 3 brackets that locate the glass and wedge it into the upper frame are marginal ( Rover did it in a far more substantial way that used threaded fittings to wedge the glass into position ). The centre bracket which relies on a single 8mm headed bolt is barely up to the task. The rubber coated steel frame across the base could well be loose ( or completely rotten ). The actual windscreen frame at the centre top ( where the stainless steel finishing strips meet and have a mere self tapping screw to locate ) could well be rotten under the stainless trims. Faulksy speaks of drips at the lower edges. I have seen wind blowing water upwards inside the cabin too as the result of poor sealing/packing around the lower frame. The "black mastic" really has centimetres of space to fill at the curves on the lower corners. The strange shaped washers can be used rather like cam lobes to help push the glass upwards into the grey rubber frame. ( There is a difference between the lesser D model black surround and the superior more upmarket/later grey coloured rubber. IIRC the actual windscreen glasses are different thickness ... no doubt Buttercup Bob will advise ).
 
My experience was eventually to pull the screen out and install with a new grey seal. Mastic behind that seal is not my preferred option as it gets everywhere. Mastic should however be pumped along the bottom of the windscreen from the engine bay side once it is installed. If this is done there should be no leaks. You need two people and a couple of hours but it’s not that hard and it is doing it as the manufacturer intended
 
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