The mirror of my RB Pro S comes back in the up position

The mirror of my RB Pro S comes back in the up position after cocking the mirror lever, from time to time with a smooth movement I was able to get the mirror in the right position but only once in a while.
Thank you.
Regards.

It most likely needs to be lubricated(CLA). When the mirror comes down a latch has to move into position to hold the mirror in place. If the latch has old lube or no lube on it's pivot it will be slow. When you hold the lever, it give the latch time to engage.

It can be one of several problems...
Latch can be worn.
slipped latch adjuster.
pin in the cocking gear can be sheared.
Bent internal parts from forcing the camera to cock, like with a locked lens on it.

When did this symptom start? was the camera forced? Was a lens locked on and someone forced it?
Most likely it can be repaired easily enough if there is no parts damage. I am certain the internal seals need to be changed anyway so doing a $50 CLA would be a worth while cost considering the camera's longevity.

paul ron, NYC
automax1@juno.com

I will be out for two weeks but I will get back to you as soon as I manage to check my RB maybe I'll get lucky. Thank you very much for all your answers. I will give you more details soon.
Regards,