Weird Brake Issue

Alright guys here is my latest problem. I'm not real sure how to explain this very well but here goes.

The first time coming to a stop after starting the engine or if its been a long time since I have hit the brakes, the brake pedal is super sensitive (maybe not sensitive for a newer truck but for a 1st gen its super touchy). As I start to apply more pressure to make the truck come to a complete stop, the pressure "breaks" for lack of a better word, and the pedal goes soft, then it firms back up and the truck stops. It's super disconcerting.

I have bled the brakes three or four times and it fixes the issue for about a week then the problem returns. I have tried engaging the abs a few times then bleeding the brakes. I tested the vacuum check valve and it ok. The master cylinder is a Dorman unit that was installed ~4 years ago. I have new extended brake lines from wheelers. I have checked for leaks and if I have one its so slow I haven't noticed it. Thought? Paying the price for not buying an oem master cylinder? Brake booster going out?

 

If bleeding the brakes repeatedly fixes the issue......its a leak in the brake system. Somewhere. That is all there is to it. Where that leak is, that is the hard part.