I really don't trust the thumb safety as all it is, is a sear blocking safety.
Great video at the URL
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Now I do have Walt Kuleck's book "The M1911 Complete Owner's Guide" and within he claims a hammer blocking function for the thumb safety. Maybe the thumb safety has been modified since 1911, but originally, the thumb safety was on the pistol only so the pistol could be safe with one hand. It was not a permanent safety, the discussion revealed in Clawson's book with MG Croizer clearly shows, it was a temporary safety. Temporary till the Cavalry Trooper could lower the hammer and put the pistol in the flap holster.
I love Kuleck's book, but he is a cocked and locked fan boy, and those guys will find things that are not really there. So, I will go with
@1911Tuner assessment that the hammer will wipe off the safety if the sear fails. The next time I take all my 1911 trigger parts out, I will try to assemble so I can hold the hammer back, with the thumb safety on. Maybe not installing the sear will allow me to test whether the thumb safety will block the hammer, or whether the hammer will simply over ride the thumb safety. It will be interesting to see.
For your pistol, if it is a series 70, the condition of those sear surfaces are critical to your safety. A series 70 does not have a firing pin block, so if the hammer, for whatever reason falls and hits the firing pin, any primer in the way will ignite. I am aware of incidents where the sear broke, and the 1911 being held fired all the five rounds in the magazine. This was in a 2700 Bullseye Pistol Match. On the load command, the match director (who was a competitor) dropped the slide for the timed or rapid fire stage. And the sear broke. All five rounds fired, and Frank had a bullet hole in the brim of his cap! Imagine where a sixth round would have gone.
I really don't know if your hammer is falling to the half cock or not. I am sure you would hear a click if that happened. So, if your hammer is falling to the half cock by pressing on it, or by pressing the trigger, you need a new sear and hammer. Preferably fitted. The half cock usually engages the sear, but not always. Shoot a series 70 1911 enough to wear out the sear, and you will be surprised when it starts doubling.