My 2005 duramax is building up too much pressure in cooling system. when i drive it the coolant either gets pushed out through the overflow or disappears into the motor. when i remove the ovreflow bo

The high pressure in your cooling system could be caused by several factors including:

Overheating engine: If the engine is running hot, it will cause the coolant to expand and increase pressure in the system.
Malfunctioning radiator cap: A faulty radiator cap can cause pressure to build up in the system and force coolant out of the overflow container.
Blocked coolant passages: If the coolant passages are clogged with debris or rust, it can cause the coolant to get trapped and increase pressure in the system.
Air pockets in the system: Air pockets can develop in the system, causing pressure imbalances and coolant to be pushed out of the overflow container.
If you are experiencing high pressure in your cooling system, it is recommended to have a professional diagnose and fix the issue to prevent damage to your engine and cooling system components.

This expansion tank is only when coolant builds up so musch temperature and pressure that it needs to release, so it relieves itself from the radiator into the expansion tank.
Then, when coolant returns back to normal operating temperature, the pressurized air contracts, forming a negative pressure or vacuum, sucking the now-cooler coolant mix back in to the radiator and cooling system.
So if your water is not getting to the bottle at all, even when engine is boiling, check the outlet and overflow line for blockages, the stem-valve in the bottle is moving, pressure/water leaks in the cooling system venting water or steam.
Also check radiator cap is correct for the car model and operating.

You have a coolant leak. The engine gets hot and pushes coolant out under pressure into the overflow tank.When it cools, the volume of coolant decreases, the pressure drops to the point that it becomes a vacuum. The vacuum pulls in air more easily than coolant. It's thinner. So, you keep losing coolant from the radiator which makes the engine run hotter, each time until it eventually overheats and does damage. You need to have the cooling system pressure tested. The leak will show up. Usually, it's a loose hose clamp or cracked radiator tank high up, which makes it had to see the coolant because it doesn't leak much and it boils off leaving little or no evidence. Fill the system completely before pressure testing. Remember, checking the overflow tank means nothing when checking coolant level. There's no short-cut to checking the coolant level, take the radiator cap off when cold to accurately check level. Read the safety precautions on the car but always check when the engine is cold. NEVER HOT.

You need to have your cooling system pressure tested. This will let you examine the engine area without the hazards of the engine running. You may have head gasket issues. Depending on the area of the leak, your cylinder compression can actually pressurize the cooling system. When the cooling system pressure exceeds the rating of the cap, the cap seal opens and can blow coolant back into the overflow bottle where it eventually leaks while driving. Check to see if your overflow bottle is full.

Leaks get worse when engine is at operating temp and pressure builds. Rent a cooling system pressure tester and pressurize the system. Then you'll see the leak

REPLACE THERMOSTAT IF TOP COOLANT COOLANT HOSE GOING TO RADIATOR IS HOT TO TOUCH THERMOSTAT OPEN. TO BLEED COOLANT SYSTEM FIRST YOU NEED TO ADD COOLANT TO THE COOLANT OVER FLOW JUG UNTIL COOLANT LEVEL STOP DROPPING AND STAY AT FULL COLD MARK.WAIT 3 MINUTES IF COOLANT DROPS ADD MORE COOLANT UNTIL COOLANT STAY AT FULL COLD MARK.WITH RADIATOR OVER CAP OFF CRANK ENGINE FOR MINUTE WATCH COOLANT LEVEL AT OVERFLOW JUG.IF COOLANT DROPS ADD MORE COOLANT IN THE OVERFLOW JUG.WHEN COOLANT LEVEL STOP DROPPING IN OVERFLOW JUG.SCREW BACK ON THE RADIATOR CAP TO OVERFLOW JUG.WHILE ENGINE IDLING WATCH TEMPERATURE GAUGE WHEN TEMPERATURE START CLIMBING TURN OFF ENGINE WAIT A WHILE KEEP FROM GETTING SCALDED.WHEN COOLANT COOL DOWN SOME. PLACE A LARGE RAG OVER THE COOLANT OVERFLOW JUG OPEN CAP LITTLE AT A TIME UNTIL PRESSURE RELEASED, THEN REMOVE CAP ADD MORE COOLANT IN THE OVERFLOW JUG TO THE FULL COLD MARK.THEN REPEAT THE PROCESS SCREW BACK ON THE COOLANT OVERFLOW RADIATOR PRESSURE CAP.THEN START ENGINE WATCH TEMPERATURE GAUGE WATCH COOLANT TEMPERATURE GAUGE WHEN COOLANT TEMPERATURE START RISING TURN OFF ENGINE AGAIN WAIT UNTIL ENGINE COOL DOWN ENOUGH TO KEEP FROM GETTING SCALDED TAKE PLACE LARGE RAG OVER RADIATOR CAP WHICH IS THE COOLANT OVERFLOW JUG CAP OPEN CAP LITTLE AT A TIME UNTIL PRESSURE RELEASED ADD COOLANT UNTIL AT FULL COLD MARK. WHEN YOUR COOLANT LEVEL STOP DROPPING.AND TOP RADIATOR HOSE VERY HOT THERMOSTAT OPEN AND AIR IS OUT THE COOLANT SYSTEM. MAKE SURE YOUR COOLANT OVERFLOW JUG HAS COOLANT IN IT,WHEN ENGINE COOL DOWN CHECK COOLANT OVERFLOW JUG KEEP COOLANT LEVEL AT FULL COLD MARK.BECAUSE IF OVERFLOW JUG IS EMPTY THATS A SIGN RADIATOR COOLANT LEVEL LOW.WHEN OVERFLOW JUG EMPTY IT WILL ALLOW AIR GET IN THE COOLANT SYSTEM AND CAUSE CAR TO OVERHEAT.WHEN REPLACING THERMOSTAT ALSO REPLACE THE RADIATOR CAP ON THE COOLANT OVERFLOW JUG.

REPLACE THE THERMOSTAT AND RADIATOR PRESSURE CAP.IF COOLANT LOW, ADD MORE COOLANT THROUGH COOLANT OVERFLOW UNTIL COOLANT LEVEL IN THE COOLANT OVERFLOW STAY AT FULL COLD MARK.REASON POUR COOLANT THROUGH COOLANT OVERFLOW MOST GM CARS DONT HAVE RADIATOR CAP.WHEN YOU GET COOLANT LEVEL OKAY, CRANK CAR LET IT IDLE UNTIL ENGINE GET HOT BUILD UP PRESSURE THEN TURN OFF CAR.CHECK FOR COOLANTS LEAKS AT THE RADIATOR HOSES,CHECK FOR LEAKS AT THE RADIATOR, CHECK FOR LEAKS AT WATER PUMP WEEP HOLE IF YOU SEE COOLANT LEAKING FROM WEEP HOLE REPLACE WATER PUMP. IF CAR OVERHEATING WHILE IDLING YOUR COOLING FANS NOT WORKING CHECK COOLING FANS FUSE AND RELAYS.IF FUSE AND RELAYS OKAY THE ENGINECOOLANT TEMPERATURE SENSOR BAD.

Have you changed the water pump ?

about 100K miles? sounds like a blown head gasket is allowing coolant to be pushed out the exaust,and combustion pressure is pushing fluid out the overflow.have your compression tested.

You may need to ask AAA to put a radiator pressure tester on the resevoir bottle and pump it up to operating pressure(Pressure cap rating) and see if it over pressurises while it is running at operating temp,rev engine up and down and watch what the testers pressure readings do,it should move up and down in sync with the water pump.If the pressure keeps building i would have to believe that combustion chamber gases are over pressurising your cooling system.Also test the pressure cap is functioning within factory specs.