Why would you lose pressure in your brake pad after changing the back brakes on a 1992 Pontiac Grand Prix?

You need to bleed the entire brake system. Bleed in this sequence, RR, LR, RF, LF. If you do not know how to do this, do a search online for "bleeding brakes" THE REAR WHEEL CYLINDERS MAY BE LEAKING SHOULD ALWAYS CHANGE WHEN DOING BRAKES AND BLEED REAR BRAKES