still trying to get my head around the downward sap factor what cause the downward pressure
So I'll jump in & then someone will be along in a minute to word it better (!) but it's all caused by
osmosis & if you understand that, then you're on the way towards understanding many natural things.
You know how a solution wants to be even all the way over - say you pour two different strengths of sugar solution into each end of the same bowl they will even out, you don't stay with areas of stronger and weaker solution.
Now presume you have a bowl with a membrane down the middle. The membrane has minute holes in it so water can go through but sugar can't (water has very small molecules, sugar has much bigger molecules). This is called a semi-permeable membrane. Pour your two sugar solutions into either side of this & they
want to do the same as before - to even out but the sugar can't move, so water does. Water moves from the weak solution to the strong one until they are the same strength. So in your bowl the side that started with the stronger solution will get more full.
Living things are made up of minute cells that act like your bowl. The cell walls are all semi-permeable membranes.
Osmotic pressure is where water comes into a cell by osmosis, so that pushes outwards (or in our case downwards). As long as photosynthesis keeps making more sugar in the plant's leaves, that will attract water in & osmotic pressure will push the sap away.