Collect 8bit data from IO ports

I'm having a problem with collecting data from Port1. the variable ValueX has a value of FF, regardless of my input through the IO pins.With here i enclose partial of my code, any help or advice is highly appreciated, thank you!

sbit P1^0 BIT0; //these 8 ports will be input
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sbit P1^7 BIT7;

signed char ValueX = 0x00;
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void ReadValue(void);
void main()
{
P1 = 0xFF; //port set high to activate as input
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}

void ReadValue(void) interrupt 0 using 0
{
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 .
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 ValueX = ValueX | BIT0;
 ValueX = (ValueX <<1) | BIT1;
 ValueX = (ValueX <<1) | BIT2;
 ValueX = (ValueX <<1) | BIT3;
 ValueX = (ValueX <<1) | BIT4;
 ValueX = (ValueX <<1) | BIT5;
 ValueX = (ValueX <<1) | BIT6;
 ValueX = (ValueX <<1) | BIT7;
}
}

i'm sure tat my setting for this ISR works because i had other function inside this ISR that works. But the ValueX doesn't seems to change. It always maintain the value FF. Any help, please.. Thanks