pump won't prime with extra hose attached

The pump is old (maybe 18 years), but worked OK last season, and have just started using it again. Now, It will work OK when the skimmer just has a strainer basket, but when a hose is attached to the skimmer the pump won't prime (I assume that's the right term - it can't fill all the way, no flow). This happens even if just a short length of hose is attached to the skimmer. Without extra hose, the pump fills nicely, there aren't a lot of bubbles, maintains about 12 psi and seems perfectly fine. Could this be a leak, or should I just get a new pump? The only obvious leak now is on the output pipe where I did a temporary (2 years ago) repair. (I need to decide whether to replace the pump before repairing that.) Really need to be able to vacuum the pool!

I'm assuming you already know to submerge the hose and fill it with a jet before attaching it through the skimmer... So, every time I have trouble maintaining prime with a hose attached, there has been an obstruction in the hose or vacuum head (usually a large acorn). That's where I would start...

Yes hose is submerged, made sure it had no trapped air or blockage. Just an 8' length of hose will be too much for the pump.

Could it be that the end of the hose is down lower than the skimmer pipe opening, so in effect the pump is trying to draw water up a greater height?