My engine heat increased on a long trip. I noticed that the reservoir tank level was above the full mark at times of over heating. my radiator is a new one. my lower radiator hose busted twice in a single day, even after replacing with a new one. A mechanic said that the radiator cap could be too tight, so he released the cap valve a bit, using a wire. to my surprise this solved the problem. could it be the only problem?

when the radiator and engine is full of coolant ans the coolant heats it expands and it sent to tank, where it is recoverd as engine cools.

The A/C causes the engine to work harder (that's why fuel economy suffers when it is run).

If your car has an electric fan - make sure it runs when the engine is hot. If it does not - you may have trouble w/o the A/C running if is gets warmer outside. Look for fail fuses, disconnected plugs, etc.

You may also have flaky thermostat that is not opening fully when needed to exchange hot water in the engine block with cooler water in the radiator.

First, check the coolant level. You should find a semi-transparent reservoir / tank located under the hood that has a cap marked "Coolant". The side of the tank should have two indicator lines - one for HOT and one for COLD. If the engine has been run up to normal temperature (so that it can provide heat in the car), you should see the coolant level at or slightly above the HOT line. If the engine is cold - first thing in the morning or after sitting for several hours - the coolant should be at or slightly above the COLD line. View these levels while the engine is off and the car is parked on a flat, level surface. If there is not enough coolant - you will need to add the CORRECT type for your car.

Hot Engine
(1) With the engine hot, add enough coolant to the tank to bring the level to the HOT line. You may have to check the tank several times after driving if the coolant level in the radiator is down a great deal. NEVER open the radiator cap when the engine is warm or hot!

or:
Cold Engine
(2) With the engine cold, add enough coolant to the tank to bring the level to reach COLD line. Additionally - with the engine cold - you can add coolant directly to the radiator by removing the cap. Start the engine, set heat in car to HOT and add coolant mixture to radiator until level comes up to the bottom of the filler neck. Once the engine is HOT (getting heat in the car) check and top off radiator as needed and put cap back on radiator.

Check the levels in the TANK several times over the next few days when cold. If the tank ever empties -you'll need to have this looked at by a mechanic.

Yes, it is important to include the overflow tank or bottle as integral with the radiator and coolant system as a whole. They all intermix. Actually, the level in the overflow tank is a true measure of the correct amount to keep your coolant at. There are two marks on the overflow: full hot and full cold. The coolant system on all modern cars is a sealed system and therefore should never require adding, save a very small amount through evaporation within the overflow tank- the only part that is open to atmosphere.

So once you get all the air out of the system that is possible, both in the engine and in the radiator, you have a full level everywhere, including the overflow tank where it should always remain somewhere between the full hot and full cold marks.

You are right that the overflow collects the heated radiator coolant. But it is due to the expansive properties of coolant-or water- that it expands when heated, (but not boiling-that is very bad news of a different sort). Your radiator cap is designed to open at a specific pressure, just enough to allow coolant to flow into the overflow. As the engine and coolant cools, it contracts, the level drops in the radiator, and a natural vacuum there will siphon the overflow tank back into the radiator until it is full again. That is why you have the full hot and full cold marks.
I will shut off the lecture now. I hope you have liked my answer. If not, I give you permission to kill me now- if I may so jest.

DID YOU BLEED AIR OUT THE COOLANT SYSTEM.TO BLEED COOLANT SYSTEM, FILL THE COOLANT SYSTEM THROUGH THE RESERVOIR TANK OR THE RADIATOR UNTIL THE FULL COLD MARK IS REACHED.BESURE TO USE 50/50 HALF WATER AND ANTIFREEZE.YOU CAN BUY ANTIFREEZE THAT IS ALREADY PREMIX 50/50.AFTER ADDING ANTIFREEZE TO RESERVOIR OR RADIATOR FILL RADIATOR TO FULL MARK OR IF YOU DONT HAVE RADIATOR CAP ON RADIATOR POUR ANTIFREEZE IN THE OVERFLOW JUG UNTIL COOLANT LEVEL STAY THE FULL COLD MARK.WAIT AWHILE UNTIL COOLANT LEVEL STOP DROPPING.NOW INSTALL THE RADIATOR OR OVER FLOW JUG CAP.START THE ENGINE AND ALLOW IT TO RUN UNTIL NORMAL OPERATING TEMPERATURE IS REACHED AND THE UPPER RADIATOR HOSE BECOME HOT.STOP THE ENGINE AND OBSERVE THE COOLANT LEVEL IN THE OVERFLOW JUG. THE COOLANT LEVEL SHOULD BE AT THE FULL HOT LINE.ALLOW THE ENGINE TO COOL UNTIL THE AMBIENT TEMPERATURE IS REACHED THEN CHECK THE COOLANT LEVEL IT SHOULD BE AT OR ABOVE THE FULL COLD LINE.ADD MORE COOLANT IF NECESSARY. WARNING BECAREFUL WHEN OPEN THE RADIATOR CAP OR JUG OVER FLOW CAP.USE A LARGE RAG OVER THE RADIATOR CAP TO KEEP FROM GETTING SCALDED.OPEN RADIATOR CAP A LITTLE AT A TIME TO KEEP FROM GETTING SCALDED.WAIT UNTIL ENGINE COOL DOWN BEFORE OPENING RADIATOR CAP OR OVERFLOW JUG CAP.WEAR SAFETY GLASSES.

It probably got overfilled at some point. There's a max line on the reservoir that you shouldn't fill it over. Coolant expands as it heats up so if there's too much in the system, it's normal for it to overflow.

It should have the level maintained in it as marked on it. If it is empty it will not cause over heating (it just holds the extra fluid) but a leak causing a low coolant level, stuck thermostat, clogged radiator, bad cap or improper pressure will.

You will have to bleed the cooling system,there is a bleeder screw near the thermostat housing,just loosen it,and pour coolant in as it comes out the bleeder,watch for air to escape the system.Then close the system back off(bleeder screw).Then refill the coolant system,and replace the radiator cap.Fill the reservoir 3/4 full past the full mark.Start the engine and let it get totally warm,but not to over heat,then turn off engine,and wait 30 minutes to see if the level in the reservoir drops,if so repeat this until the reservoir level stops dropping,and the cooling fans turn on,and turn off by them selves,with the defrost off,heat on.

The cooling system has not been completly full of coolant,make sure the level of coolant drops,when the engine heats up,and the thermostat opens up,then the coolant level will drop,when it does,be ready to fill the radiator completly full at this time,then cap the radiator off with the radiator cap.Be sure to fill coolant reservoir up,a little past the full mark.so it can fill it self as needed.If there is not enough coolant in the system,it will do what it is doing now.

Did you open the radiator cap and make sure the radiator is full of coolant? Sometimes the hose from the coolant reservoir to the radiator becomes plugged, and the radiator will become low on coolant even when the plastic reservoir is full. Low water levels will prevent the coolant system from getting hot water from the engine to the radiator or heater cores. This will cause the engine to run hotter while the heater appears to not be working.

it seems like the radiator of your car needs attention. check for coolant level in it, when its cold. if les add some coolant and then see if the problem persists.
--than there are chances of chocking in the coolant pipes, may be they not reached to engine..  
--Check the chock,, that it is not turned on. 
--May be sum loosing nut in engine.