This may sound silly but have you considered adding some type of "big toe" you the foot of your robot to help with balance?  Think of the human foot. Without out toes we fall on out face. 

I've never built a biped robot but I see other people attempt it. Mostly builders make the feet of these robots very big to help with balance. DJ's JD is an example. I've often thought if a set of toes (or one big toe) powered by servos and controlled by a balancing PID script would help with keeping a robot upright. It would go a long way to help with balance and give a more proportionate looking foot.  Probably much more involved and expensive then the average builder would want to try. You would need a motor controller capable of reading feedback and PID control (like a Kangaroo or a RoboClaw), feedback devices, just the right kind of servos and a lot of time, effort and luck to adjust the PID settings. 

Maybe there's a simpler way.

Thanks for your input Dave!

Funny enough, I've had a few people mention the same sort of thing about making the feet bigger. Going back to the beginning of this build I knew that the original Voltron feet weren't going to be sufficient enough to allow him to walk, so I ended up hacking in JD's feet. This allowed Voltron to be able completely balance on one leg, Voltron doesn't have an issue with balance, he can stand for long periods of time on one foot. The issue that a 3D printed robot of this size faces is 3 fold: rigidity, height and weight. You see, as DJ was describing if you place a weight on the end of a stick, depending on how heavy the weight is, the height of the stick, and how much flexibility the stick has will determine the amount that the stick will bend. In Voltron's case the chest weight is pretty heavy, it's tall (~16 inches), and the legs are flexible. 

Last week I tried to address the weight factor. I tore Voltron apart, @Nomad described it as open heart surgery, and I hollowed out all the material I could without losing structural integrity. I don't know if it was worth it as I only saved 20 grams, but what's done is done.

This week I am addressing the flexibility and height factors. I have designed and 3D printed new servo brackets for the legs. The new brackets are printed in PLA, which should be more rigid than the original ABS brackets. The brackets also shrink the overall height by about 1cm per bracket. 

These adjustments will make Voltron shorter, more rigid, lighter, and bring down Voltron's center of gravity and in theory make it easier to walk.

Would it help if you just made the feet more heavy? This is what Wow Wee company did with the really tall V2 robots, by having D size regular batteries in the feet it gave enough weight to allow many different walking gaits. Then people started using lighter Nimh batteries which would make them start to fall over many times unless more weight was added to the feet. In my case with the tall bot I am simply using a lead acid wheel chair type battery to prevent fall overs as I have the room to add one at the bottom.

@Robo Rad Yes, more weight on the feet may help, but the flexibility and height factors need to be addressed first.

Congrats Jeremie you did it ,Voltron now lives to save the Earth!

Edit:   You mentioned that new robot movie"Alita Battle Angel"... I did watch that right after on my win10 Kodi program it is already there in the Supremacy new releases and it is a high quality cam actually...Insane robot battle Action,great movie!

I had meant to make a post last night but I must not have hit "post comment", my bad.

Thanks @Robo Rad! It was a great night, I'm very happy that Voltron now walks! I really appreciate all the support from this community with cheering me on all the way to the finish line. 

For those who don't want to watch the 12 hour struggle (3 Hack nights worth), you can see a tiny clip on my Instagram Page. Thanks to @DJ for his help filming a small clip after our hack session, when he was ready to head home.

With this win it makes me very hopeful that we can see all kinds of creative 3D designed Robots being printed, come to life, and walk away. I am a true hardware guy, and always like to talk about the hardware side of things, but the real secret sauce of Voltron comes from the ease of use of the software: EZ-Builder. I don't talk about it enough. If not for the real time adjustments I could make with EZ-Builder, I could not have made the walking gait of Voltron. 3D printed robots, built with a lot of hot glue, are far from precise and they need software that can adapt, EZ-Builder does a wonderful job of that!

Thanks again Synthiam Community, I look forward seeing to the next robots you all are making!  

P.S. @Robo Rad I'm glad you liked that movie, I hope to see it soon.