proportioning valve Need more rear braking power

[Suburban_Tyler]

I have a 1990 R1500 2wd Suburban. The rear brakes need more power to them. I tried tightening the shoes and bleeding the brakes. When I have a trailer hooked on it gets down right scary trying to stop. Thats with about 1,500lbs behind the Suburban and not a whole lot of tongue weight. If the windows are down I get hear the tires starting to slip. I can literally get better rear brake power if I push the parking brake down. Is the a different proportioning valve I can get that would do better as far as getting more power to the rear brakes. Normal driving it stops fine. Always has a good pedal. If I have to make sudden stops you can count on one or both of the front tires locking up depending on the surface.