I replaced the water pump and radiator in my 93 audi 100. Both top and bottom radiator hoses get hot and pressurized, but the water seems not to be circulating, and will overheat within about 15 minutes...any suggestions?

The upper coolant hose, or top hose, connects the engine to the radiator whereas the bottom hose connects the radiator to the engine, commonly to the water pump.

The top hose is the hot water outlet for the engine and is generally connected to the thermostat or similar housing.

There are variations in terminology and the best way of proceeding in identifying parts is to use a repair manual for reference or ask an Audi Parts Department to either print off a diagram or to email or fax it to you.

sounds like the thermostat may not be opening.

This can be due to one of a few different things...First of all, water flows from the engine to the bottom of the radiator, using the water pump and gravity, and the water returns from the radiator back into the engine on the top side.

Thermostat isn't opening...this causes the water / antifreeze to remain in the engine, and not flow through to the radiator. If the water / antifreeze gets hot enough, it should activate the thermostat and water should flow into the radiator through the bottom radiator hose.

The bottom radiator hose is either worn out (there is a spring inside the length of the bottom hose. it is there to keep the hose from collapsing under the suction pressure during normal use) If the hose feels spongy, soft, or shows signs of cracking, replace it.

The radiator may be clogged, preventing the water from flowing freely through all of the 'fins' in the grill section of the radiator. Hot water usually isn't just in the top section during a clogged radiator situation...

Check the radiator-it may have blockage. Take it off and turn upside down and flush from the bottom.
If the top hose gets hot, that means the thermostat is opening. The bottom hose should be at least warm, and quite warm if radiator flow is good.

What about the water pump if the water pump is working right there would be pressure somewhere.

COULD BE THERMOSTAT IF TOP RADIATOR HOSE IS NOT HOT AND BOTTOM RADIATOR HOSE GOING TO WATER PUMP IS HOT THE THERMOSTAT STICKING PARTIALLY CLOSE.IF TOP HOSE IS HOT THE RADIATOR,ENGINE BLOCK HEATER CORE NEED FLUSHING OUT.IF WATER PUMP WEEP HOLE LEAKING REPLACE WATER PUMP, FAULTY WATER PUMP WILL CAUSE ENGINE TO OVER HEAT.REPLACE RADIATOR CAP MAKE SURE YOU HAVE 50 / 50 MIX SOLUTION ANTIFREEZE AND WATER IN COOLANT SYSTEM. IF ALL LOOKS GOOD CHECK ENGINE OIL ON DIP STICK, IF OIL IN DIP STICK LOOK LIKE MILKSHAKE YOU HAVE BLOWED HEAD GASKET THAT WILL CAUSE ENGINE OVERHEATING WHILE DRIVING CAR IN SHORT DISTANCE AND ENGINE OVERHEATING.

Here is a basic description of cooling system, the top hose going to engine is the supply hose the bottom hose is the return hose. On your overflow coolant reservoir there is a lower cold level mark and a hot level mark above it also there is an overflow drain spout several inches above hot level mark to relieve pressure if system is overfilled. As to a bleeder system, bleeders purge air from a system to allowed it to become primed (pressure built up) generally on hydraulic systems like brakes or pneumatic systems like a diaphram pump for example. the only drain available is the petcock valve at the bottom of the radiator used to drain coolant. Now to your top hose being empty, you can remove top hose to engine have a garden hose running into radiator and if radiator cores are not stopped up water should flow freely from hose if not radiator is clogged. If you have water flow now you can pressure test system it should hold between 15-18lbs pressure constantly otherwise check all hoses/clamps etc (if leaks were present before replace faulty component/s) your vehicles water pump circulates coolant and is what pressurizes cooling system. A faulty thermostat will also restrict coolant from entering the engines coolant passageways if it doesn't open and your vehicle would overheat soon after, but you should still have coolant in upper hose when engine is running regardless. If you can add other info like your vehicle is running hot or coolant light coming on I can narrow it down for you.

REPLACE THERMOSTAT AND RADIATOR PRESSURE CAP SEE WHAT HAPPENS.

change thermost and/or water pump

When you mention the radiator having a hot upper in pipe and a cool bottom out pipe tells me that the coolant is not moving around the engine. This could be down to a water pump not working, blocked radiator, thermostat not opening. The bottom hose should be cooler than the top hose but once the engine is up to working temp which is around 85/90 degrees c for an audi the bottom still should be farly hot to touch. Hope this helps