What is this clutch part? Pressure problems

[javatrooper]

I 've got a 90 trooper. Lately it's been getting a little harder to shift. Years a go I had the master cylindar replaced. It appeared this started to leak a little so I just replaced it and bleed the system. For some reason, I can't get the pressure up enough. I've bleed the bleeder at the slave until I have new clear fluid and no air coming out. I've also bleed the bleeder at this strange unknown connector that appeares to have a diaphram on the side of it. It is connected to the hard line coming off of the master cylindar. I'm wondering if this part could be the problem?

Either that or my soft hose is going bad, but it appears to be Ok, but has never been replaced. My slave cylindar seems to be fine. What I can't understand is that I could at least drive my car with the old master in it, but now I can't even get the pressure fully up even through pumping on it multilple times. I keep coming back to that diaphram thing. Maybe there is air trapped in it somehow. BTW, the clutch is only one year old, I dropped in a 3.4L from a 95 Camero last year. Lots of power!

What you think?