Horse opening its mouth when reins pulled on hackamore?

It is the predictable reaction to pain and pressure that results when the muzzle is basically being put into a vice. The desired response to pressure from a leveraged mechanical hackamore is submission (backing off of or away from the pressure with the mouth closed) and flexion at the poll. When rein aids are combined with body aids as they should be, the desired rate, suspension, or collection may be elicited with momentary. light engagement of the otherwise loose reins.

Misuse of the hackamore and/or insufficient training will lead to the horse exhibiting resistance to the pressure, which produces extension of the poll and raising of the head with the mouth gaped open to relieve the vice effect forcing it shut. Remember that the jaws are hinged well back behind where the lips meet, and a tightened hackamore clamps the mouth shut.

A mechanical hackamore is a powerful leverage device that functions similarly to a curb bit with a gag effect, applying vicelike force against the chin and nose instead of the chin, tongue and bars of the mouth that a bit applies.

The noseband and curb chain form a loop around the muzzle that attaches to rigid metal shanks at each side to function as lever arms. When the reins are engaged, the shanks rotate backward, (counter-clockwise) which forces the noseband down against the nasal bone as the curb chain tightens upward against the mandible (chin bone), trapping nerves and other soft tissues in between. If the reins are over-engaged, then inside of the mouth, portions of the tongue or tissues of the cheeks may become trapped and pinched against or between the teeth as the mputh is forced tightly closed by the "vice", while superficial nerves and vessels in skin are painfully compressed.

I recommend retraining the horse to respond to body aids, either in a snaffle bit (no leverage shanks).or in a Dr. Cook bitless bridle which is designed to painlessly distribute pressure mildly and evenly over a large surface area. Unlike leverage devices with shanks, either the snaffle or Dr. Cook designs can be used for delivering direct, independent lateral rein aids, which is necessary in order to train a horse to respond to body aids. The horse has to be correctly adapted to whatever bridle is used, which requires expertise on the part of the trainer, but the mechanical hackamore should not be used as it has been since it is obviously eliciting resistance from the horse.

He avading the contact! in other words he doesnt like it, by opening his mouth he's relieving pressure and pain. I would just take it off him.

The most likely thing that could be causing the problem: pain due to a problem in the mouth. For example, bad teeth. If his teeth have sharp edges or points that need floating, then the pressure holding his mouth closed could be pressing those into his gums, causing pain and injury. He evades the pain only by opening his mouth. An abscess or other lesion could cause this as well, as could many other mouth problems. This could also explain why she's been having problems with the bit.

It could also be a problem with the hackamore. Is it adjusted correctly?

The horse wouldn't be opening his mouth to evade the hack unless it helped. It could only help if the hack's adjusted wrong. It may be that he's changing the angle of the leverage between the nose piece and the curb change to shift the pressure to a more comfortable place.

The nose band part should be pretty high on his face - there's a little spear of bone that defines the bridge of the nose, down towards the nostrils:

http://glaciervet.com/Hook%20Skull.jpg

The hack needs to be high up on that bone, or it can actually be broken. The curb chain has to be properly adjusted as well. Here's another diagram, showing the fit of a bitless bridle - the hack should be adjusted similarly:

http://www.bitlessbridle.co.uk/images/fitting-skul...

These are both simple suggestions, easy to check out yourself if you know the owner and can talk to her. Hope they help.

If you can ride with a hackamore you might be able to get away with using a halter with your reins clipped to the sides. I ride my mare like that and she's fine.

The reins on a hackamore shouldn't be pulled. The reins on anything shouldn't be pulled.

The horse is likely to be evading the face pressure any way he can. If I'm not mistaken, mechanical hackamores have curb straps so it's likely he's opening his mouth to relieve this pressure.

Poor horse, it just needs some proper training and knowledgable owners.

Hackamores in the wrong hands can be very severe and can place immense pressure on to the sensitive areas of the horse's face/nose. It's also possible that even with light contact that the horse simply doesn't like the different pressure, not all horses like hackamores or bitless bridles.

As you mentioned though the owner bitted up excessively prior, so the issue is more than likely her.