Simpson Eziset 950 - Water overflows when filling, tube is clear and solenoid is working. How do you clean the water level pressure chamber? Do you have to remove the bottom black cap on the chamber?

Always disconnect power & turn off water before doing any work.
This is the simple fix for Error code E20
Simple fix to this problem with out having to buy another controller board.
Carefully remove back cover &
Make a lite adjustment to the electronic pressure sensor
turn with Philips driver 45 deg clockwise to raise water level by 25mm will fix the pump running all the time & will spin now.

I am not familiar with the model but if the spare parts, if any are needed, are available all faults can be fixed.

There are two big questions - what has gone wrong and how much will it cost to fix and is it worth it.

Leaking water internally is a rather vague description. I assume you mean the washtub is filling when it isn't supposed to?

If it is clean fresh water it could be the solenoid fill-valve that is leaking or alternatively is receiving current when it shouldn't.

If the water is grey, dirty, soapy water it is probably a syphon-back situation that sometimes happens when the machine discharge is badly connected and the required air-break is blocked.

there is a small hose from the bottom of the tub to the water level switch
that gets air pressure to activate a switch when the level selected overcomes the switch spring
At that point the water solenoid is turned off and the wash cycle begins
over flowing indicates that the level sensor is not working or the inlet solenoid and diaphragm is faulty
if it doesn't progress the wash cycle , that indicates a program board problem so call the technician for a proper diagnosis

check the small tube to the level pressure switch
check the switch is working
check/replace the water control solenoid / diaphragm
tests are
with out clothes
select a low level wash
listen as the water flows in
when the tub is about 1/2 full the water solenoid shouf make a clunk sound and the water cuts off
at the same time the wash cycle begins
if the water comes right up and there is no solenoid sound then check the small tube or the water level switch
if there is the sound but the water continues replace the diaphragm in the solenoid

there is a small tube running from the bottom of the tub up to a pressure switch in the control panel that is set by what ever wash level you want
when the pressure in that tube exceeds the pressure set by the knob it does 2 things
it turns off the inlet water solenoid to stop water coming in and then starts the wash cycle
as it is starting the wash cycle , it indicates that the stop solenoid for the water inlet is either still getting power or the stop valve is faulty
try replacing that water control valve first

2 items to check using a multi meter check water pressure switch and water inlet valve

see pics
Washer fills with water and overflows:

The water pressure level switch is a pressure activated switch that is used to determine the correct water level in the tub. This part is normally located behind the control panel and will have a hose or air dome tube that connects it to the tub. On washers that use an electronic control, this switch may be located at the sump area below the tub. As the water fills in the tub it will compress the air in the tube and push against a diaphragm inside the switch. The switch will control the water inlet valve and turn the valve off at the correct water level. If the tube has a leak, not enough pressure may be created to activate the switch and the washer may overfill. If the switch is defective it may also cause the machine to overfill. To determine if the switch is bad, you should first eliminate the air dome hose as a problem. Look for signs of cracks or worn areas on the hose. Remove the hose and submerge in water. Seal one end and blow air into the other end and look for bubbles. Check for an obstruction in the hose as well. If the air dome hose appears to be ok, then the water level switch may be defective.

You can test it with a meter and with a manual test? ( first locate it in the panel, it has a plastic clear tube hose going into it, remove that tube from its connection, now put another small tube or hose into the slot where it was and have the machine set to wash cycle with water in it. Blow into the other end of the tube.
If the machine starts to wash. the pressure switch is more than likely good
( No wash- then THE DIAPHRAGM IS BAD AND SWITCH HAS TO BE REPLACED).

The other item to check would be your water inlet valve. Check it for continuity with a multi meter it should show Ohms or a closed circuit. To test it when water is overflowing unplug the unit and see if it still overflows or continues to fill. If it stops when unplugged. Then it is not a water inlet problem but an electrical signal problem going to ur water inlet valve. Possibly from the panel or board. You can also do a live volt test to see if it is getting the full 120 current, but be sure to unplug unit before testing as this is a live test.
Also another trick to check the water valve, is to first cut the water off and remove the hoses off the back of the washer. Then one at a time point the hoses inside the washer tub and turn the water back on. If you don't have good water pressure, turn the water back off and remove the strainer that should be in one end of the hose. Inspect the strainer to see if it is stopped up, clean it and reinstall it. Now repeat this test. If you have good water pressure now, reinstall hose and your problem is fixed

By the way my advice is free cuz God is good!

Washer fills with water and overflows:

The water pressure level switch is a pressure activated switch that is used to determine the correct water level in the tub. This part is normally located behind the control panel and will have a hose or air dome tube that connects it to the tub. On washers that use an electronic control, this switch may be located at the sump area below the tub. As the water fills in the tub it will compress the air in the tube and push against a diaphragm inside the switch. The switch will control the water inlet valve and turn the valve off at the correct water level. If the tube has a leak, not enough pressure may be created to activate the switch and the washer may overfill. If the switch is defective it may also cause the machine to overfill. To determine if the switch is bad, you should first eliminate the air dome hose as a problem. Look for signs of cracks or worn areas on the hose. Remove the hose and submerge in water. Seal one end and blow air into the other end and look for bubbles. Check for an obstruction in the hose as well. If the air dome hose appears to be ok, then the water level switch may be defective.

You can test it with a meter and with a manual test? ( first locate it in the panel, it has a plastic clear tube hose going into it, remove that tube from its connection, now put another small tube or hose into the slot where it was and have the machine set to wash cycle with water in it. Blow into the other end of the tube.
If the machine starts to wash. the pressure switch is more than likely good
( No wash- then THE DIAPHRAGM IS BAD AND SWITCH HAS TO BE REPLACED).

The other item to check would be your water inlet valve. Check it for continuity with a multi meter it should show Ohms or a closed circuit. To test it when water is overflowing unplug the unit and see if it still overflows or continues to fill. If it stops when unplugged. Then it is not a water inlet problem but an electrical signal problem going to ur water inlet valve. Possibly from the panel or board.

This is a seriously wild stab in the dark at your problem. Washers usually have a totally jacked-up schematic somewhere in them. I never can figure them out.
But, I think your fill switch is stuck. The ones I've worked on use a type of pressure switch where water rises in a tube and makes the switch.
Find the switch and see what it is doing. The washer thinks water is still in the tub because the switch is telling it so.
Clean the lint trap also.
Our KenMore washer also automatically selects warm and it ain't broke.

Water-inlet valve A defect in the water-inlet valve may mean that it's no longer able to shut off completely when the electricity has been turned off to it. If this occurs, the valve may leak and drip water into the clothes tub. In time, the water may accumulate substantially. If this happens, you need to replace the valve. Water-level switch A defect--or an obstruction--in the water-level switch may mean that it can't tell the water to shut off. So the machine overflows. This switch senses the water level in the clothes tub. It's usually a diaphragm device with a small, clear tube attached between the switch and the bottom of the washer's outer tub. As the water level in the tub increases, the pressure on the air in the tube increases. When the pressure reaches a certain level, it activates the switch, shuts off the water, and signals the timer to begin the agitate cycle. You can either clear any obstruction in the tube or replace the water-level switch.

If it fills with water and starts the cycle and the water keeps running in then your water inlet valve on the back where the water supply comes in is sticking. It needs replacing.

If it overflows and don't start the cycle-
Your pressure switch stops the water and allows it to start the next cycle. It's a switch either in the control panel or at the top of the tub with a small tube connected to it which goes down between the inner and outer tube. As the water level increases the air in the tube is compressed until there is enough pressure to make the switch stopping the water from filling. This tube can get clogged and the switch won't work. Remove the tube and clean it out. Blowing on the tube when it's still connected to the switch will make the switch if it's working and clear.