RB67 Mirror Lock Up not working

I'm putting my second roll of film through my RB67 Pro-S and am having MLU issues with the only lens I have (65mm-C). I had no trouble with the MLU working with the first roll, but now it seems every 3rd shot or so pushing in the "normal" shutter release on the camera will also set off the shutter. I can reproduce it with the back off the camera or even just using the lens and manually cocking it and it's very random when it happens. If I pull out the MLU knob on the lens just a little more it always works as it should. I wonder if the spring on the MLU knob is weak and not engaging the mechanism correctly?

Any ideas before I send this back to KEH? Especially since they seldom have 65mm-C in stock.


Edit: If I screw a cable release into the lens and move the MLU knob 1/2 between off/on where it's pulled out the farthest the MLU system works every time.

You can send it back and requests service rather than replacement...

Or use it with cable release as you describe.

If it is in good condition optically Id not let it go.

My experience w/ KEH (great company) is that they will attempt fix your lens, not swap it for another. If it's within their 14 day of purchase I would think they will refund your money, minus shipping. Call to ck. So you might want to weigh that vs just using the lens w/o MLU. If it works as it is w/ a cable release it may not be a big deal, as I would imagine any shots using MLU would be on a tripod anyway.

I would contact KEH just to see what they maybe able to do. I just a couple months back had an intermittent shutter release on MLU too. I'd only had the lens for a few weeks when I contacted them. KEH had me return it for a CLA and it is working perfectly now.