The channels in the GMC radiator core are the only way heat can be efficiently dissipated from the engine. The channels have hundreds of fins attached to them that actually released the trapped heat into the atmosphere so a constant temperature can be maintained by your power plant. The radiator core is fed hot coolant from the radiator hose situated on the top, while the exit hose is located on the bottom for the return of the coolant to the engine. For this reason the flow is downward through the radiator core and its channels for the cooling process to take place on your GMC truck. An annual flushing of this coolant system has to include the removal of the sediments that have built up on the channels of the radiator core on your GMC vehicle. The sediment is from the dissolved solids that are present in water used in the coolant system. The only way to help prevent this type of sediment buildup in your GMC radiator core is to use distilled water when mixing the coolant of your system. Most vehicle owners find it more cost effective and easier to just flush their coolant system annually since that process is necessary independent of the type of water they use. If the sediment is allowed to accumulate too much, then the size of the channels will be reduced removing the ability of the radiator core to dissipate heat from the coolant. It can also cause a leak to form in the core over time. This will increase the cost of repairs to the GMC owner immensely with the replacement of the radiator itself or scorched bearings in the engine if it overheats for a prolonged period of time.