The port pins on these processors are not designed with active pullups. Instead there is a weak pullup device on the chip that makes output pins stay high when they are written to a "1". If you are seeing the chip output only able to rise to 150 mV is is because the weak pullup on the chip pin cannot supply enough current when it is in the high state.

I looked at the data sheet for an Atmel T89C51RB2 and noted in the electrical specification section that ports 1, 2, 3, 4 & 5 can supply current out of the pins in high mode up to 60 microamps while still keeping the output pin at 3.5 volts (Vcc=5V).
If you pull more current out the pin voltage will go down more.

The port 1 pins have much higher pullup current ratings. These pins can supply up to 7 milliamps while keeping the pin at or above 3.5 volts (Vcc=5V). If you pull out less current and these pins will rise even higher.

So,,,,since you need 0.7 milliamps to your load you should re-consider connecting this load to a port 0 pin.

Hope this helps
Michael Karas