Do you mean when you press the altitude HOLD button (under the Altitude Indicator)?
Or do you mean it does not hold the altitude set in the Altitude Indicator when descending either using VNAV or VS?

I often, when using VNAV or VS to descend, press the HOLD button to maintain altitude until capturing the GS. My 757 will HOLD the altitude until it does capture the GS when I use it that way.

Mark

Hmmm. Not good. I wonder why? It isn't trying to capture the Glide Slope is it?

Have you tried pressing HOLD when climbing, to see if it continues to climb instead of holding? Just curious to find out if it happens that way too.

Mark

I usually stay on VNAV and LNAV until I capture the ILS and GS.

A simple example:
My last waypoint (SOSIJ) before capturing the ILS and GS has an altitude constraint of 4000A, the next waypoint (SY07), the last one before RW34L has an altitude constraint of about 1900, but I'm not worried about that because the G/S keeps me at/above that altitude at that final waypoint. So. I have set the Altitude Indicator to 4000, the goal is to be at (or close to) 4000' upon reaching the waypoint (SOSIJ). The 757 will be at the altitude I need when it gets there, so I would usually press the LOC button on reaching the waypoint (SOSIJ), followed by the APP button just prior to the GS pointer reaching the center of the ADI.

I have, on some occasions, pressed the HOLD button, when I have found that the GS pointer is not far above the center of the ADI (as long as I have already captured the ILS) followed by pressing the APP button. For me, the 757 will hold the altitude until the GS is captured.

Mark