I've recently replaced the thumb safety lever on my Colt 1911A1. The problem with the older safety lever was that it would not "hold" at times and with the thumb safety on and the pistol firmly gripped in your hand, on pulling the trigger the thumb safety would occasionally drop and the hammer would fall, as if there was no safety engaged!
This problem has disappeared with the new thumb safety lever, however another problem seems to have surfaced - on engaging the thumb safety it is now impossible to disengage it unless one uses ones nail to first depress the safety lock plunger and simultaneously use one's thumb to depress the thumb safety!

It seems like I obviously need to stone or file the lever in some way to enable smooth operation. My question is - what would be the correct method to follow and how much and where all do I file away?
I have recently also replaced all of the springs (except the recoil spring & firing pin spring) and pins in the gun with new ones (using a Wolff rebuild kit). Don't think it should have any bearing on the above problem, but mentioning it anyway, just in case one of the new springs could be the cause of the problem...
The new thumb safety lever used is on this page - part no. 560-453-601
Cheers!
Abhijeet