What makes the water reservoir all of a sudden start to boil real hard and warp the top lid that screws on it? how can we fix it?

If you're talking about the model shown in your picture, the leak is most likely between the water reservoir and the tank. Bunn uses a soft silicone seal for that connection that gets compressed over time and starts to leak water whenever you make a pot of coffee.

It's planned obsolescence. Bunn won't sell you a new seal. So you have to improvise. But it's simple enough to do.

Remove the top cover and the plastic water reservoir right below it. Don't tamper with the little valve that opens when you close the lid.

Look for the seal. It could be stuck to the bottom of the reservoir, or sitting loosely on top of the metal tank. Make sure to preserve its top-to-bottom orientation (don't put it back in upside down).

Apply a narrow bead of clear silicone into the grooved side of the seal. Press the seal gently onto the reservoir, then apply a thin layer to the smooth side.

Re-install the reservoir and reassemble the unit.

Does it do it just when you stop ?

The leak is most likely between the water reservoir and the tank. Bunn uses a soft silicone seal for that connection that gets compressed over time and starts to leak water whenever you make a pot of coffee.

It's planned obsolescence. Bunn won't sell you a new seal. So you have to improvise. But it's simple enough to do.

Remove the top cover and the plastic water reservoir right below it. Don't tamper with the little valve that opens when you close the lid.

Look for the seal. It could be stuck to the bottom of the reservoir, or sitting loosely on top of the metal tank. Make sure to preserve its top-to-bottom orientation (don't put it back in upside down).

Apply a narrow bead of clear silicone into the grooved side of the seal. Press the seal gently onto the reservoir, then apply a thin layer to the smooth side.

Re-install the reservoir and reassemble the unit.

hello nikeea,not sure what car/truck we are talking about 07 doge or 94 cutlass??
in either case if the problem started after radiator replacement,it sounds like you have air trapped
in the system. if you look on the thermostat housing(at the end of top radiator hose that connects
to the engine) there maybe an air bleed screw to bleed out the air. open screw with engine warm & idling until you only have fluid coming out,close bleeder & top off system

NOTE; some modles will have an additional bleeder around the water pump area
leave the heater on high while bleeding system

Seals could need replacing. If your fairly handy you can replace them.

Here are the seals;
http://iwantsweetdeals.com/Keurig-Replacement-Water-reservoir-Seals--P1286283.aspx

You can purchase a replacement reservoir here;
http://iwantsweetdeals.com/Keurig-B40-B60-Water-Reservoir-Tank--P1207341.aspx

Try taking the top off the unit (3 screws) and remove the pour-in tray (1 bolt) and turn the thermostat down. Replace the tray - making sure you put the silicone washer in - put the lid on, and try it again. If this doesn't cure it, your thermostat is bad, and you need to replace the unit.

I had the same issue, poured in 8 and out came 6. Here's what I did to clean and fix.
1) Unplug the NHB
2) Remove accessories like the pot and basket
3) Bring NHB over to the sink
4) Snap off the entire top cover (not the swing lid) by starting at the front and lifting upward (the top cover is about 1/4" thick). The back of the top cover will pivot a bit and then pop off exposing the pour reservoir. Remove the flow stopper inside the pour reservoir by lifting it straight up.
5) Place the top cover and flow stopper aside.
6) Remove the brew diffuser (located underneath the top where the brew basket goes. Twist counterclockwise gently but forcefully enough to snap it past the lock tabs which hold it in place. You'll know that it you've done this correctly when it just falls out.
7) Still over the sink, tip the NHB in a fashion where the back side is facing upward and the brew side is facing down into the sink. You'll be holding the NHB horizontally by the top and bottom of the unit. This will allow any water in the tank to pour out the brew spout. Begin to tip the unit as if tipping upside down as this will allow any remaining water to drain out. If this does not work the brew spout hose may be clogged and you can just tip the unit entirely upside down to empty it from the fill hole in the pour reservoir.
8) Return the NHB to the upright position in the sink.
9) Using a scotch brite sponge and some baking soda, clean the inside of the pour reservoir and rinse.
10) Using the tool Bunn provided, clean out the brew spout hose which is located under the brew hat. This is the little hole where the water comes out. If you don't have the tool Bunn provided you could use something like a plastic tie-wrap. Don't use anything metal as the hose you're cleaning is made of plastic tube and could become punctured. Clean this hose for just a minute or two to ensure that there are no significant obstructions.
11) With the NHB upright in the sink, pour a slow stream of hot water from the faucet, about two cups, directly into the fill hole located in the pour reservoir.
12) Tip the unit horizontally, as in step 7, and place the tip of your finger over the pour reservoir fill hole covering it completely.
13) Shake the unit back and forth horizontally so the water in the tank splashes around and frees anything which may be built up in the tank. Some water may splash out of the brew spout hose.
14) Tip the unit as in step 7 to drain the tank water out.
15) Fill a 16 ounce glass with hot water and 2tbsp baking soda, mix and pour into the pour reservoir fill hole.
16) Repeat steps 13 and 14 once.
17) Repeat steps 11 through 14 three times.
18) Clean the pour reservoir cover, swing lid and flow stopper.
19) Put the flow stopper back in place.
20) Attach the top cover.
21) Clean the exterior of the NHB.
22) With the boil and warmer switches off, plug the NHB into the electrical outlet.
23) Turn the boil and warmer plate switches on.
24) Fill the NHB with one pot of cold water and close the swing lid. Wait one minute.
25) Fill the NHB with another pot of cold water. Place the pot on the warmer plate and close the swing lid. If water does not yet come out, fill with one more pot of water.
26) Allow water to drain from brew spout until it stops draining.
27) Empty pot.
28) Fill pot to 10 cup line and pour into the NHB. You should find that the NHB now delivers 10 cups back to the pot. Empty the pot.
29) Clean brew spout diffuser and replace back in the brew hat.
30) Clean the pot, pot lid and brew basket.
31) Wait ten minutes for the water in the tank to boil.
32) Brew your coffee as you normally would.

The only issue I experienced during this process was with the warmer plate switch. It appeared that the light in the switch blew although the warmer plate still warmed normally. Shortly after the water in the tank started boiling the warmer switch light started operating again.

I had the same issue, poured in 8 and out came 6. Here's what I did to clean and fix.
1) Unplug the NHB
2) Remove accessories like the pot and basket
3) Bring NHB over to the sink
4) Snap off the entire top cover (not the swing lid) by starting at the front and lifting upward (the top cover is about 1/4" thick). The back of the top cover will pivot a bit and then pop off exposing the pour reservoir. Remove the flow stopper inside the pour reservoir by lifting it straight up.
5) Place the top cover and flow stopper aside.
6) Remove the brew diffuser (located underneath the top where the brew basket goes. Twist counterclockwise gently but forcefully enough to snap it past the lock tabs which hold it in place. You'll know that it you've done this correctly when it just falls out.
7) Still over the sink, tip the NHB in a fashion where the back side is facing upward and the brew side is facing down into the sink. You'll be holding the NHB horizontally by the top and bottom of the unit. This will allow any water in the tank to pour out the brew spout. Begin to tip the unit as if tipping upside down as this will allow any remaining water to drain out. If this does not work the brew spout hose may be clogged and you can just tip the unit entirely upside down to empty it from the fill hole in the pour reservoir.
8) Return the NHB to the upright position in the sink.
9) Using a scotch brite sponge and some baking soda, clean the inside of the pour reservoir and rinse.
10) Using the tool Bunn provided, clean out the brew spout hose which is located under the brew hat. This is the little hole where the water comes out. If you don't have the tool Bunn provided you could use something like a plastic tie-wrap. Don't use anything metal as the hose you're cleaning is made of plastic tube and could become punctured. Clean this hose for just a minute or two to ensure that there are no significant obstructions.
11) With the NHB upright in the sink, pour a slow stream of hot water from the faucet, about two cups, directly into the fill hole located in the pour reservoir.
12) Tip the unit horizontally, as in step 7, and place the tip of your finger over the pour reservoir fill hole covering it completely.
13) Shake the unit back and forth horizontally so the water in the tank splashes around and frees anything which may be built up in the tank. Some water may splash out of the brew spout hose.
14) Tip the unit as in step 7 to drain the tank water out.
15) Fill a 16 ounce glass with hot water and 2tbsp baking soda, mix and pour into the pour reservoir fill hole.
16) Repeat steps 13 and 14 once.
17) Repeat steps 11 through 14 three times.
18) Clean the pour reservoir cover, swing lid and flow stopper.
19) Put the flow stopper back in place.
20) Attach the top cover.
21) Clean the exterior of the NHB.
22) With the boil and warmer switches off, plug the NHB into the electrical outlet.
23) Turn the boil and warmer plate switches on.
24) Fill the NHB with one pot of cold water and close the swing lid. Wait one minute.
25) Fill the NHB with another pot of cold water. Place the pot on the warmer plate and close the swing lid. If water does not yet come out, fill with one more pot of water.
26) Allow water to drain from brew spout until it stops draining.
27) Empty pot.
28) Fill pot to 10 cup line and pour into the NHB. You should find that the NHB now delivers 10 cups back to the pot. Empty the pot.
29) Clean brew spout diffuser and replace back in the brew hat.
30) Clean the pot, pot lid and brew basket.
31) Wait ten minutes for the water in the tank to boil.
32) Brew your coffee as you normally would.

The only issue I experienced during this process was with the warmer plate switch. It appeared that the light in the switch blew although the warmer plate still warmed normally. Shortly after the water in the tank started boiling the warmer switch light started operating again.

The Bunn coffee machine has an internal tank used for holding approx three potfulls of water at just below brewing temperature. When you add water for a potful, the thermostat senses cold water and starts the brew heater. The water level inside the tank comes up and goes through the sprayhead. This "surge" of water, that you call drips is normal. The hot water stratifies to the top of the tank, just like an electric water tank in a house. The internal pot heater kicks in and boils the water already in the tank, raising the water level in the siphon pipe to the sprayhead until the reservoir under the add water opening is empty.