Bosch SGS69A02GB/17 water level problem

Can anyone advise on how the water level in the reservoir (inside the left panel) is controlled? My dishwasher fills the reservoir to the top OK for the initial pre-rinse cycle, goes through the rinse and wash, heats, pumps out OK etc. When it comes to the rinses it only takes a small amount of water, about 1/8 of the reservoir which it then tries to circulate but it is not enough to rinse properly. The timer eventually runs down to 1 minute and sticks there with the machine still trying to rinse with a tiny amount of water.
My question is – how is the inflow of water for the rinses controlled? Should it fill the reservoir to the top for each rinse or does it take a smaller amount of water for rinses? If the latter, is the inlet valve opened for a fixed time which is set by the control board? I am trying to figure out whether the control board is faulty or maybe the inlet valve (or something else that I have not thought of!). Thanks to anyone who can shed light.