I just assembled my first lower and when I function tested the trigger, before putting the upper on, it failed to reset the trigger and hold the hammer back unless I released the trigger. Is that the disconnector? 

How do I see what's going on inside? Thanks, I appreciate the help.

Updated Question

First, thank you Dano523 for the videos, it allowed me to diagnose my problem. Now the question is "How do I fix it?"

It appears that my disconnector is not functioning correctly. As long as I maintain pressure on the trigger the hammer will not reset. If I am not mistaken that means the gun would runaway if I tried to fire it. Is that correct? How do I get the disconnector functional?

Thanks again.

Thank you!

Updated the OP.

What you are saying is how it is supposed to work. It won't reset until you let off the trigger.

Pull trigger - click/fire
Let off trigger - click/reset

Thank you. I think that's it.

10-4.

Probably not the right wording. The hammer would not catch the disconnector.

That sounds better
Reset happens after the disco grabs the hammer, when the disco drops it to the trigger sear.
Your disco just isnt't grabbing it and you would have hammer follow. This can be bad juju. Real bad juju.

The disco spring upside down is a common one.
One end is bigger than the other to hold it in there for some reason. Not sure what they had in mind there, it's not like it can get out of there once assembled.
If you put the big end on the top, it will hold the spring compressed instead of pushing the disco fwd. This opens up the overlap between the disco and trigger sear and lets the hammer slip through.

If you put it together correctly and it still does it, you can file the disco fwd of the pin where it comes down on the front of the trigger to give it more overlap.

I had one disco that was a little tight (well, a lot of tight) in the trigger slot for some reason. Not sure if it was the trigger slot, burr on the disco, or disco too thick.
I just narrowed the disco up with a piece of sandpaper on a flat surface until it fit nicely.

If you swap the bolt catch spring and disconnector springs, (usually) you will not get correct disconnector function. Or if the disconnector spring is upside down.

If your Grip Screw is not standard (too long), then you could also run into a similar problem, by the grip screw pressing on the trigger assembly.

Just saying......

As long as you maintain pressure on the trigger, the hammer SHOULD NOT reset, that's the whole purpose of the disconnector.

Or do you mean the hammer just returns to forward/fire?

(do not let the hammer slam unrestrained into the lower receiver during the following if you're doing it with just the lower)

Cock hammer
Pull trigger - hammer should fall - HOLD TRIGGER TO THE REAR
Recock hammer - hammer should click onto and be retained by the disconnector
Release trigger - hammer should click forward a small amount and now be held by the trigger

Engage safety
Pull trigger - hammer should not fall

If the hammer is not being restrained by the disconnector, then yes you have a problem.

One common issue when people assemble lowers is swapping the bolt catch and disconnector springs. If you put a bolt catch spring in the trigger, generally the disconnector will not work right. Another common problem is putting a disconnector spring in upside down, so that the flared part is outside the trigger - the flared part should get pushed down into the trigger.

With the hammer forward, push back on the disconnector, it should be free to move back and forth under positive spring pressure.

Or you could have a broken or out of spec disconnector and/or hammer. Do you have any spares that you could try swapping those parts? Could also have an out of spec lower receiver such that the parts do not have the right geometry to function correctly.


Something extending into the receiver from the grip screw hole will act to pull the trigger, by pressing upward on the tail of the trigger.

Some use a set screw in the grip screw hole just for this purpose, as a takeup adjustment. Using a setscrew in this manner allow removal of excess takeup to the point of engagement with the safety. There are also lower receivers on the market with a separate screw for this function.

I have a similar problem.  When I do the function test with the lower only and release the trigger, it does not reset. The trigger will move forward but not enough to reset.  I can manually move the trigger forward the rest of the way and then it will reset.  It does not feel real smooth as I move it forward.  Does it sound like the trigger spring is too weak?

Or maybe just not installed correctly.

I'll check it.

Figured I would touch base and thank everyone for the discussion.

Missing disconnector spring was the culprit.

It checked out OK.  The disconnector had a burr.  Filed it off and it works great now.