I replaced the rear brake pads on my Saab 9-3. Now the brake pedal is soft on first push, firm on the next. This repeats after driving just a few seconds.

If the brake pedal was ok prior to changing the brakes that shouldnt be the problem unless you have disconnected or opened a brake line. If the rear calipers are the wind in type and the park brake is part of the caliper, most likely the pistons are not lined up correctly with the caliper housing

You will need to bleed the brake system. I would start with the rear brakes lines and work towards the front.You must have key off engine off when bleeding brakes, apply pressure on brake pedal 10 times and hold. The brake line is now ready to be open. Open and let the stream of fluid come out and close the line tight. Then release brake pedal once bleeder screw is close. This procedure must be repeated twice front and back. After, brakes have been bleed pump the brake pedal before putting in drive.
Note: Don't let the resevoir run low on brake fluid. This will cause air bubbles to accumulate on the brake system. Always top off before opening bleeder screw.

YOU NEED TO COMPRESS THE CALIPER PISTON USING SPECIAL TOOL 09580 - 3400 OR YOU CAN USE A LARGE C CLAMP AND THE OLD BRAKE SHOE NEXT TO CALIPER PISTON.BE SURE TO SIPHON OUT 1/3 BRAKE FLUID OUT OF THE MASTER CYLINDER BEFORE COMPRESSING CALIPER PISTON BACK IN THE CALIPER BORE.WHEN YOU FINISH REPLACING REPLACING REAR BRAKE SHOES.MAKE SURE BRAKE MASTERCYLINDER RESERVOIR COVER BACK ON RESERVOIR BACK IN PLACE.THEN PUMP BRAKE PEDAL TO SEAT BRAKE SHOES AGAINST ROTORS AND UNTIL YOU GET FIRM SOLID BRAKES.THEN CHECK BRAKE FLUID LEVEL.DONT DRIVE CAR IF BRAKE PEDAL FEEL SOFT NOT FIRM.CHECK FOR LEAKING BRAKE HOSE OR LEAKING CALIPER PISTON SEAL.

Sometimes a caliper can stick, making that brake be engaged all of the time. The reason it will get worse as you drive (and the burning smell) is because of friction that is created. You need to start by finding out which side the caliper is locked up on, and replace it. Usually it is only held on by two bolts. Since you have been driving around on it that way, you will need to replace the brake pads on both sides, and the rotor on the side that is locked up (although it's best to do those in pairs too). When you get the new caliper installed, you have to bleed the new caliper by having someone push the brake pedal as you slightly unscrew the bleeder valve on the caliper. You should use a small hose connected to the bleeder valve and submerged in a small glass container of brake fluid so that you can watch for air bubbles. When the fluid and air stops coming out, tighten the valve, have the assistant release the brake pedal, and repeat until no more air comes out. Don't forget to keep the master cylinder under the hood full of brake fluid.

The rear brakes on the Spectra will always engage less than the front brakes. If you have to push hard on the brake pedal then you probably have air in the brake lines from the last brake pad replacement. The brakes need to be bled with this method: Use 1/8 inch 2 foot long hose from parts store and attach from bleeder valve ball on the brake caliper to a bottle partially filled with brake fluid. End of hose should be submerged in bottle. With second person holding brake pedal to floor, slowly turn bleeder valve counter clockwise (loosen tiny bit) with wrench and watch for dirty brake fluid to create bubbles in the bottle. When bubbles start, quickly tighten bleeder valve back to tight. Second person lifts foot while valve is tight and then pushes pedal to floor again and THEN you again open bleeder valve slowly turning counter clockwise and watch for bubbles. Repeat until no more bubbles can be created in the bottle where the hose is submerged. Move to next caliper and repeat. For a firm brake pedal you will need to repeat this on all four calipers. My 2003 Kia's rear brake pads "fell apart" and never wore out in 190000 miles because they do not get used. The braking power is primarily in the front disk brakes on this model. Removing and replacing the rear drum brake linings did not even require me to bleed the brake lines. They weren't that tight before or after the replacement procedure.

If you have a soft brake pedal and have to push hard to slow down or pump the brakes to slow down then you probably have air in the brake lines from the last brake pad replacement. The brakes need to be bled with this method: Use 1/8 inch 2 foot long hose from parts store and attach the hose from bleeder valve ball on the brake caliper to a bottle partially filled with brake fluid. The end of the hose in the bottle must be submerged in the fluid in the bottle. With a second person holding the brake pedal to the floor, slowly turn bleeder valve counter clockwise (loosen tiny bit) with wrench and watch for dirty brake fluid to create bubbles in the bottle. When bubbles start, quickly tighten bleeder valve back to tight. Second person should lift foot, pump and then while valve is still tight should push the pedal to the floor again and THEN you again open bleeder valve slowly turning counter clockwise and watch for bubbles. Repeat until no more bubbles can be created in the bottle where the hose is submerged. Move to next caliper and repeat. For a firm brake pedal you will need to repeat this on all four calipers.

you have air in the system or the master cylinder is going bad. Have a shop inspect and bleed for safety reasons if you can afford it.

Try having the dealer flush and bleed your system. It is very hard to bleed ABS systems yourself and have safe brakes that still work afterward. Bleeding non-ABS brakes yourself is easy not the same for ABS brakes. Valving, sensors and what-not require a tech and the correct equipment in my opinion. You do it wrong and you could ruin your ABS system. Do that and see if they firm up. I replaced my brake shoes/pads at the same time all new everything in back, drums/springs everything and new rotors up front. then I had the chevy dealer flush, refill and bleed system. Stiffer pedal and brakes work better. Keep in mind the brakes on 99 Tahoes are inaedequate, require new rotors often, heat up and fade/glaze pads regularly. I replace my pads long before they wear down because they glaze up and start fading early. I'll rough em up once maybe, next time, new ones. Every two brake jobs, new rotors for me. Just how it is. They will stiffen a bit and work better but they will never be awesome brakes. Just how it is on 99 and earlier Tahoes. Hope it helps. Very important to bleed correctly though. I'll bleed my 83 Toyota 4x4 myself but not the Tahoe.

bleed the system

Have you put the correct caliper on each side?
Are bleed screws facing upward?
If so you will probably need to have brakes pressure bled to push through air in ABS system.