All good advice.
Painting the housing to reflect the light outward will help brighten things up, if you're going back to an incandescent bulb. Check the condition of your lenses as well as boatpuller suggests.
If you stay with the 1157 replacement LEDs, painting the housing shouldn't make that much of a difference since the light output is directed outwards at a specific viewing angle. As evan mentioned, reputable LED suppliers like
www.superbrightleds.com offer a wide range of 1157 LED cluster configurations. Check out the superbrightleds site and maybe upgrade to a larger cluster of LEDs for more brightness.
Like Zoomad and the others said, check the wiring. Here's what I have on my current setup using the stock incandescent bulbs. (Light output for the tail lights and brake/signal lights look normal.)
................... Tail Light ........... Brake/Tail Light
.......................................................................
Resistance ... ~2 ohms ............ ~ 3 ohms
Engine Off ... ~11.5 V .............. ~ 12.0 V
(Battery)
Engine On .... ~14.2 V ............. ~ 14.7 V
(Alternator)
BTW, I have a trailer harness plugged into the wiring.