My BUNN GRX-B is leaking water when brewing. I think it is out of the back?

if the coffee pot does not work right, take it back and get another one...

not brew if no water coming in to filll the tank

This is really a home style VPR brewer (but looks nicer)
if you listen to their video (link below) they mention it has a pre-heated tank of water already in the brewer. This means the brewer needs to be pre-filled with a full pot of water and pre-heated prior to using the brewer. Unplugging this unit won't be a problem UNLESS it was emptied of tank water and not refilled when powered up.
* The unit will try and heat up the tank of water but if empty then just air which overheats the unit then blows the thermal fuse. Call for service.
* The warmer is still working but it's not hot enough to brew frequently as the warmer may take up to an hour to get the tank hot enough to brew or it may not get hot at all if it's own thermal fuse blew too.
* Water leaking while the brewer just sits there? Leak in the tank. Call for service.
* water leaks only while pouring? Then your pouring to fast. Wait for the water to drop before pouring in more. If it still leaks even while pouring slowly then there is a leak in the fill tray. Call for service.
* Final; call for service to be safe.
Aloha, ukboy57
https://www.amazon.com/BUNN-Velocity-10-Cup-Coffee-Brewer/dp/B00EAQJCWW/ref=sr_1_1?ie=UTF8&qid=1516431654&sr=8-1&keywords=bunn+nhs+velocity

Maybe limed up.Try running a deliming spring in the brew tube.Bunn p.n. 1111.0000.

Your brewer is still under the 3 year warranty. If you will call us at customer service at 800-352-2866 we will be happy to send you out a new machine.

If you will give us a call at 800-352-2866 we will be happy to further assist you with this issue.

It sounds like the tank isn't full. It takes 2 full decanters of water to fill the tank in the bank. After pouring in 10 cups of water, close the lid and wait 3 min. Then open up the lid and pour in another 10 cups of water and close the lid. Then turn the vacation switch on and wait 15 min. Now you are ready to brew coffee. Place your grounds and filter in the funnel basket and pour in 4-10 cups of cold water and place decanter on warmer plate. Close the lid and in 3 min you will have a pot of coffee. If this doesn't happen, please call 800-352-2866 and we will be happy to assist you.

If you will call 800-352-2866 we will be happy to further assist you in getting your brewer to work properly.

Complain loudly to Bunn, they fixed mine for free.

there's probably a 'silicon rubber' hose that feeds hot water to the 'grounds basket'- it may be loose, or damaged. is it leaking water, or coffee?? we have a bunn 8-cup, which has a pre-heated water tank that brews a pot in about 3 min. energy waster, but when you need coffee, you need coffee!!! is your filter holder properly centered? is the carafe lid hole clean and un-obstructed?
if it's leaking water, and not too costly to replace- you may consider replacing it with a new one!
good luck!