I am a bit confused after reading your post. You say that "I open radiator cap some water comes out due to pressure built in it". But you do not mention if the engine is HOT at the time of opening the radiator cap or if you allowed the engine to cool off prior to opening the cap. Herein lies my confusion:
First confusion: If the engine is hot AND you are opening the cap, you are obviously not familiar with the operating principle of the cooling system. You NEVER NEVER NEVER open the cap on a hot engine unless it is an “emergency” and that too is done with EXTREME care because the water is under 0.9 Kg/cm2 of pressure and you run the risk of injury by scalding hot water shooting out to burn your face hands etc.
Second cause of confusion: If I assume you know the operating principle of the cooling system and are conducting an experiment of some kind wherein you are opening the radiator cap with full precautions of personal protective equipment while opening the cap. If this was the case then you would have mentioned some other observations and you would not have said that there is pressure in the radiator. So I am inclined to think that you don’t know the operating principals of the cooling system. I believe that if you know the operating principal of the system then you will be ia better position to trouble shoot. So here goes:
The system operates in the following manner:
The cooling system is a "PRESSURE" system. It is semi sealed. When the engine is cold, the thermostat valve is closed. As soon as you start the engine, the thrmostat valve directs the water to circulate within the engine block only (we will call this short circulation). This causes the engine to heat up quickly to about 85 degrees (that temperature is stamped on the thermostat valve - cooler valves are also available 75 or 80 degrees etc.). Once the thermostat valve opens, it stops the water from circulating within the block and directs all the water flow to the radiator (we will call this long circulation). As the water heats further it naturally expands and in doing so pressurizes the cooling system until it reaches a pressure of 0.9Kg/cm2 (this pressure is marked on your radiator cap). At this pressure the valve within the cap opens and excess pressure (or water) is released into the expansion bottle. The cooling system reaches a steady state (temperature) indicated by the steady place of the needle on your temperature gauge. When the car is tuned off, the water cools, the pressure drops as the water contracts until it actually goes into a state of low vacuum. When that happens, there is anther valve within the cap (not the pressure valve that opens to allow water out at 0.9 KG/cm2) that opens under vacuum and allow water back into the radiator from the expansion bottle.
A point already mention in another post, the presence of the thermostat valve is essential to the proper operating of the cooling system. When there is no thermostat valve, not all the water passes through the radiator (remember the short circulation?). In fact a fair amount of water keeps circulating within the block never going to the radiator to be cooled. Without the thermostat both short circulation an long circulation are occurring simultaneously. That is bad because the water in the short circulation never gets a chance to get cooled via the radiator. If you do intend to remove the thermostat then you must block off the short circulation path.
After the above explanation, if we go back you your observations, we can say it normal for the water to be under pressure and come out of the radiator. Having said that if you suspect that your car has suffered over heating and blown the head gasket, then if you remove the radiator cap on a stone cold engine, start the car and see bubbles (similar to boiling water) you most certainly have a blown head gasket. Sometimes the gasket damage is so slight that it becomes apparent after the engine has reached its operating temperature.
If you are experiencing water loss and there are no apparent leaks under the car, what about the heater core leaking into the car? Any wet carpets? If all is well and there are no leaks, then your cooling system, is not sucking back the water from the expansion bottle in an efficient manner. Possible cause as already mentioned is the cap. Genuine cap should always be fitted.