First, thank you for the forum and the instruction videos. You provide a service well beyond your membership.
I purchased a Marin MBX-50 for my child. Everything for the setup went smooth until the brake adjustment. The rear sidepull caliper brake was not aligned and one side was rubbing. I spent hours on the bike adjusting the brake and wheel, without success. The bike shop I purchased from on-line (no lbs sells Marin), told me it was the easiest brake known to man and I should bring it to my local bike shop. In all fairness the tech told me to take a brake and return to the work after a rest. I spent hours going through videos on line and could not get the answer I needed. I learned more about brakes and bikes then I could ever imagine. I finally figured it out and wanted to share my instructions and "pass it on" to the next guy. This is not for the professionals here, but for someone out there like me who is searching for this answer.
May I tell you what I feel would have helped me in adjusting the brakes? These are instructions I drew up myself. I hope if you are encountering centering problems that this may help you. My brake is labeled "Alhonga", and it is basic. It does not have allen bolts. There is one nut in the rear, one in the front and a front locking bolt. I used only a crescent wrench for this job.
1. Take off the locking cap nut in front of the calipers. This is the one with the rounded top.
2. Loosen the nut that was underneath the locking cap 1/4 to 1/2 way to the end of the bolt. You will notice the calipers opening and there is wobble when moving the two ends of the calipers in opposite front to back directions at the same time.
3. Loosen the rear assembly nut found on the back part of the assembly attachment to the frame, slightly, and align the brake pads even. Tighten this nut securely.
4. Tighten the front nut so there is no wobble in the calipers but not enough to prevent movement during the squeezing of the brake. During this adjustment, only one of the calipers may show movement with squeezing the hand brake. This is ok. Do not worry about centering at this point. The next step will center the calipers.
5. Take the locking nut cap and screw it back on. Screw it down moderately tight to the front nut until the edges of these two front nuts approximate. Tight but do not overtight.
6. This is the most important part to understand. Centering adjustments are now made by placing your wrench across the space where the two front nuts meet (turning the locking nut and front nut both at the same time with your wrench) moving either clockwise or counterclockwise to center your calipers and pads with the wheel, even on both sides.
7. Test your hand brake.
8. If there is problems with opening of the calipers, then the front nut is too tight. Remove or loosen on the front locking cap and then loosen the front nut slightly. Replace the locking cap and secure it down and test your hand brake once more. You may need to adjust centering of the calipers by moving the locking cap and nut together at the same time slightly in either direction together as above.
9. The brake pads may have to be adjusted at this point to contact the rims.
These are instructions I made and this is what would have helped me. I hope this will help other dads or moms. For me the missing part was understanding that adjustments are made by turning the locking cap and front nut at the same time. Had I had these instructions it would have saved me much frustration. I'm not sure if this is 100% correct, but it worked for me. As this bike comes with no instructions at all, it was the best I could do.
BTW I do not begrudge the time put into this bike when seeing the excitement and smile it brings to my fortunate 4 year old.