Sounds like you have a load on the pin that is just plain too low of impedance. The uC pin can only source a small amount of current in the "pullup" state (This is the reason they are referred to as weak pullups.) For example if you were trying to drive a pin load of 100 ohms to a high level and only seeing a 1 volt high level a quick calculation shows that the current draw in the load (i=v/r) is 1/100 = 10 mA. There is nary an 8051 derivative on the planet that can source 10 mA on a pin when it is operating in the quasi-bidirectional weak pullup mode.

Buffer your load with some kind of external circuit. If you look here you can see one way that a port pin can be buffered to support a very low impedance load.

http://www.8052.com/forumz/read.phtml?id=53043

Michael Karas