biting when being petted

Question
Our 16 month old Mastiff boy, Gus, began biting when being petted at about a year old. He used to love to be massaged. I have tried to follow the 'exercise, discipline, affection' rule, but suddenly last fall he would turn and give a very hard bite on the hand when he was being petted. We live in an isolated area of a game preserve. Last summer I was walking Gus and his sister Mokie when they saw some deer near the road and pulled and dragged me. Because I injured my hand I didn't walk them for a while. I began letting them run free. Gus Mokie is 90 pounds and Gus is 2 times her size. Now I am having a really hard time. Even walking them around here, besides the deer, elk and all there are nothing but hills. I get sooo worn out before they are tired, and they can pull away and overpower me if they want.
. I used to get Gus to lie down and submit when he would nip, but he challenged me and I got nervous once he became full grown, and he knew it. My husband and son have not played an athoratative role with the dogs. My husband only wants to pet them and then only if he is not too tired when he gets home, my son was somewhat teasing and aggressive with Gus. My son came to live with us when they were almost a year. One day my son raised his voice to me and was yelling and Gus attacked him. Now it is really crucial for me to get this dog rehabilated and all of us here on the same page. My vet referred me to a local trainer and I am going to be talking to her soon though we really can't afford to hire help, I don't know if we can afford not to. I don't belive that Gus is a bad dog, I don't want to get rid of him. He and Mokie play really well and I love them both LIKE MY KIDS. I want to get a back pack for them to weight them down and wear them out. I need more games to interest them, and I am praying my husband will build me a yard, since I have thousands of open acres.Please tell me anything to help! Right now I am not petting and begging for no one in the house to yell at me or the dog. It is too tense and I really want to have things get better. I am using a muzzle with Gus but he acts like he would like to kill me when I put it on. Thanks Nancy for listening, I really liked the advice you gave on the website and I hope you can help me.

Answer
Laura,
This dog could kill someone.  I don't know if he can be rehabilitated without a trainer.  Make sure the trainer is one that works with police dogs and aggression.  This is a very dangerous situation.  What is worse is that if this dog turns on one of you again, he could lead the other dog to do the same and you will have a dead family member.  If the trainer says he trains with treats YOU DO NOT WANT HIM/HER!  This dog needs serious training by someone who is not afraid of him.  And you cannot afford NOT to do this.
In the mean time, you need to get the other dog (who seems ok) under your power.  While one is away getting trained, you must train the other so he is completely bonded and obedient to you.  Watch Cesar Millan or buy his tapes.  Do not use these methods on Gus because you need help with Gus.Gus is in the dominant role in the house and will be a danger until you are in the dominant role.
Also, get some pepper spray and keep it in several places in the house as well as in your hand whenever you are dealing with this animal.  A blast in the nose works miracles if they get out of hand.
Also, you may want to call the breeder and see what they suggest.
Nancy