is there some kind of proportioning valve for the slave cylinder on a 89 pick-up 22RE 4wheel dr. 5 speed?

 

There is a proportioning valve for the brakes. There is a rod connected to your rear axle and is attached on the upper right side of the truck under the bed. As the truck bed is loaded down with weight, the distance from the rear axle to the proportioning valve changes. This changes how much "power" is going to the rear brakes. The proportioning valve is separate from the slave and master cylinders on the brake.
That is what a proportioning valve does. And yes, you do have one. However, it is not connected to the slave cylinder.

 

xcfunrunner...I never thought of the weight factor being added in there and causing it to adjust. Great explanation on that. Thanks!!