I have a Model 19-4 2 1/2" That I recently started shooting. Most of my shooting is double action but at 25 yards I decided to try single action. The single action I found to be very light (hair trigger). Examine the single action closer I found "Push Off".
Wound this be caused by the trigger, or hammer, or both ?

Usually it's the SA bevel in the trigger that is worn or has been modified in some way. Could also be caused by damage, crud or wear to the cocking notch on the hammer. If the SA measures below approx 3 pounds, that's an indicator of a problem.

First, check the rebound slide spring to see if it has been cut or is a "lighter" weight aftermarket spring. Many times, just installing a full length OEM rebound spring can solve the problem, and bring the pull weight back up to 3 pounds. An altered mainspring or mainspring strain screw can also contribute to this.

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Ret. LE, FA Instr, S&W Armorer

The last five Smiths I’ve purchased (all early post-war N and K frames) all exhibited push-off, one to a dangerous degree. Each one had a field-modified rebound spring with two, three, or four coils having been clipped. Replacement of the clipped rebound springs with OEM springs cured the push-off condition in each of the revolvers. After I replaced the first two or three, I ordered a dozen new OEM rebound springs.

Installed a 14#trigger return spring. Push Off is more pronouned

A S&W OEM spring would be approx 18 pounds. If a heavier rebound spring (heavier than 14 pounds) doesn't help restore the trigger pull to around 3 pounds in SA, then there may be other mechanical issues.....like a damaged (dull) or altered trigger bevel, for instance.
The trigger bevel can usually be sharpened and fully restored to service in the field with the right tools and procedure.

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Ret. LE, FA Instr, S&W Armorer