I just installed a new caliper on the passenger side rear and my braking still sucks.

I am used to the quick and responsive braking from a Lexus GS350 and honestly expected more from the G's brakes.

Could it just be cheap pads?

When I stomp the brakes I expect an abrupt stop instead I seem to be having a quick slowdown with little help from the ABS...
Help!!

What pads are you running?

The oem pads are actually pretty good and have better bite than most of the aftermarket pads out there.

The 03 G has world class 60-0 braking. 110 ft. You are probably on some aftermarket pads that don't dust but give up brake performance. Switch to OEM pads

Could it be anything other than pads that could cause this? Like the Master cylinder or anything??

I am putting new pads on tomorrow.

Thanks
-jarv-

It could be the master cylinder, I suggest bleeding the lines if you haven't already.

Also, when was the last time the brake fluid was changed?

Sounds like you have air in the system. Does the pedal feel spongy? I'd change or at least bleed your fluid before changing your pads. It's much cheaper

When you install new calipers, youre either supposed to change both the front or both the rears at the same time .. changing one will result in uneven wear and bad performance.

Change them out in pairs or not at all

List of things that might affect braking performance:

Hard Tires
Worn out shocks
Brake pad material OEM vs aftermarket
dirty brake fluid