Rear brake light bulbs

My rear brake light bulbs seemed to burn out at the same time. I noticed last week when I did my preflight check that there was no rear light or brake light from either and my Skene P3 lights lit but did not flash brake either. Upon inspection today I find continuity on both bulbs but the post contacts burned off to the point they don’t make contact as you can see in the photo of one good turn signal bulb and one bad brake light bulb which are the same part number. The contacts inside the bulb housings look burned as well. Is that the cause or something else at play here?

First thought is, that looks like a combination of vibration and/or current:
Vibration can cause a constant (or occasional) "make and break" of the electrical continuity between the leaf of the socket and the solder that's used as the tip of the bulb.
Over time, this can physically wear down either part, but the solder (being tin and lead) typically wears faster than the copper (not pure, probably a "half-hard" alloy to add life and springiness).
Then, every time there is a make-and-break event, the filament cools down a little bit... Then when continuity is re-established, first, the initial input current flow - called Inrush current - is higher than the "typical running" current due to the filament's lower temperature and therefore lower resistance. This stresses the filament (repeatedly) and shortens its life.
Additionally, there is also often a small spark at the point of contact (bulb tip to socket leaf) - this also wears down both parts prematurely, exacerbating the problem. The lower socket in your photo looks worse here, but that may just be the color & shadowing of the picture.

You might try gently prying the leaf contact upward just a little, to try to keep it in more constant contact with the bulb tip; also examine the taillight mounting and see if if something is causing or allowing vibration or bouncing around.
Make-and-break can also be caused by an intermittent wire or connection anywhere along the path, on either the +12 or the return side of the circuit. Tap and jiggle any wires and connectors that you can reach while watching the bulb, maybe something will actually rear its ugly head.

Thanks for the help and for clarifying my thoughts. Should be able to fix it now.

Maybe related. On my R1150R I installed a Kissan Signal Minder which had the option for full-time running lights. The center contacts on the bulbs eroded so rapidly that I was changing bulbs every 6 months or so. I turned off the running light feature and the contact erosion stopped happening. Those turn signal and brake light bulbs are designed for intermittent, not constant use. Maybe better bulbs would bake a difference.

Thanks Paul. These are stock bulbs from MaxBMW and the rear light fixture uses 4 of the same bulb. Two are turn signals which I haven’t changed in 5 years and the contacts looked clean. Two are running/brake lights and like you said because they burn all the time they don’t last as long. What I am finding though is the filament doesn’t burn out but rather the solder tip at the end burns off. The spring contacts on all 4 were depressed more than they needed to be which is easy to do if you put too much pressure on the bulb when you insert it and press down to lock. I sprung them up quite a bit after cleaning then pushed the bulbs in very lightly until just enough to twist lock. In another 6 months I will check and see if the contacts I cleaned look better or not. At least that should rule out any of the problems suggested earlier. Wiring is all sound as far as I could test it.

mikegalbicka said:

What I am finding though is the filament doesn’t burn out but rather the solder tip at the end burns off. .

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That is exactly what I found with the signals used as running lights by the Kissan Signal Minder. The center contact just seemed eroded with some pitting. The contact points in the sockets looked OK but the center tip on the bulb didn't.

If the 1156 and 1157 were designed for "intermittent, not constant use", then we'd see a whole lot more cars with no taillights.
I've had an 1157LL in the rear of my 1150RT for many years with ZERO filament burn-out issues. I have 1157LLs in my front turn signals, ditto.

Pauls1150 said:

If the 1156 and 1157 were designed for "intermittent, not constant use", then we'd see a whole lot more cars with no taillights.
I've had an 1157LL in the rear of my 1150RT for many years with ZERO filament burn-out issues. I have 1157LLs in my front turn signals, ditto.

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Filament failure isn't the issue. The issue is the +12v bulb (center) contact eroding until contact is lost or intermittent. It only happened to a dozen or so such bulbs over five or so years until I turned the running light function off. It was/is not my imagination. Exactly why I don'tknow but I can tell you the running light function from a Kissan Single Minder kills turn signal bulbs. Believe it or not.

I have found that dielectric grease on the contact points help.
OM

mikegalbicka said:

Two are turn signals which I haven’t changed in 5 years and the contacts looked clean. Two are running/brake lights and like you said because they burn all the time they don’t last as long.

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Of course on the Hexhead/Camhead, the "all the time" running is at 5 volts rather than 12 volts, so not a direct comparison to the addon running lights.