hi...what is the saiz of the pelampung of the main tangki?

What do you mean by 'pelampung'? Is it the float?

if you refer to the 3rd photo in 1st page, the black ball on the left is the float. The white gadget is the intake valve that can shut the water flow.

Both are sold separately. Normally, only the intake valve is replaced... only one standard size. To be sure, you measure the pipe diameter, and buy the intake valve to fit the pipe.

Note that the white one in the photo is plastic... you can get a better one made of brass - about RM 25.

The black ball or float, it is attached by turning the ball onto the end of the rod.

yes the float...now have to check out the tank for pipe diameter...brass valve is better? thanks for the recommendation

I am four years too late, lol. An easier way for TS to check is to
1. shut off water supply,
2.drain empty the tank,
3. open back the water supply
4. lift the float to simulate full tank level

if water comes in from the outlet line, it means the check valve is passing.

All check valves will fail at some point in time. The either fail open or stuck in closed position.

Usually there's nothing wrong with the float, and the old one can be reused.

The brass valve is more 'heavy duty'. Worn out rubber rings/seals would caused slow leakage... but if the thin rod, which the float is attached to, broke...

If there's leakage and overflow, and you need to shut the main water supply because the intake valve is faulty, you can remove the valve and take it to the shop.

This way, you break the job into 2 sessions...

The pipe network could be simpler and fail-proof without using any check valves.

The water flow from the mains should branch at ground level, one going up to the tank, the other to the kitchen area at the back of the house.

The connection of the water from the mains to the outlet pipe from the tank is done so that the same tap can have water from either the mains or the tank? It doesn't make sense...

This to me is a clever idea. When the mains pressure is higher than the pressure in the tank (which comes from the liquid height), it will supply water to the bathroom (or whichever tap that is taking water from the tank).
When the water pressure is low, or supply is cut, the water from the tank will flow to the bathroom tap. We all know the water from the tank can be rather low in pressure, which makes installing rain shower impossible.
A second check valve should be installed along the pipe supplying water to the tank so that the water in the tank does not drain down to your neighbour's house via the mains

The disadvantages of such set up are:
1. Water in the tank is hardly replenished since our mains pressure is/should always be high (in an ideal world).
2. Usage of check valves as a form of safeguarding is never a good idea, especially in a area as inaccessible as the tank under the roof.