I use a grease gun on the fittings about once per season.

Do the bearings have an escape overfill area if you install too much grease?

I am always scared because I was told if you add too much grease you will blow out the seal and ruin the bearings.

Any help would be appreciated.

No escape. If you over fill it you can/will blow the rear seals.

If you pack your own bearings be sure you pack them correctly. You must force the grease down into the bearings. I do this by putting grease in the palm of one hand, grab the bearing with the other and slap the edge down into the grease until you see it start to exit the other side of the retainer. Make sure you rotate the bearing to get the grease packed completely around the bearing.You cannot just rub it on the outside of the bearing, on the face of the race, and expect it to do the job. It's referred to as packing the bearings.

Good luck

I use a grease gun on the fittings about once per season.

Do the bearings have an escape overfill area if you install too much grease?

I am always scared because I was told if you add too much grease you will blow out the seal and ruin the bearings.

Any help would be appreciated.

Bearing Buddy or EZ Lube Hub?

Why are you adding grease?  Do you have an indicator that shows a need for grease?  It would behove you to check your seals.

I haven't added any to mine since I redid them 4 years ago.

burly is correct in asking what system you have. If it is EZ Lube, you can pump as much as you have time and grease to do it. The excess goes either into the axle or out of the end of the spindle depending on the axle type you have.