Are the rear brake components tight and are the shoes installed correctly.
If the leading and lagging shoes are installed backwards, the shoes will grab and skip along the ID of the drum. The whole truck shudders and the rear end hops up and down.
My previous truck had this problem (same as yours - 86 Sierra Classic 3/4 ton). Kind of felt like an ABS system on steroids. At certain times, when the brakes were applied, the whole truck would shudder and bounce. It was real bad.
It got to the point where I had to figure it out. Long story short - whenever I changed rear brake shoes, I just replicated what the first owner had done. It was done wrong - he had the lead and lags reversed.
Once they were corrected, the truck braked smoothly - never did the bunny hop thing again.
The following is from a GM advisory on fundamental brake problems to their dealer technicians (this is the section that pertains to grabbing):
REAR DRUM BRAKES
Causes:
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•Shoes/Linings incorrectly installed. Each side should have a leading and trailing shoe. The lining surface on the trailing shoe will be slightly larger than the lining surface on the leading shoe. The trailing shoes should be installed in the rear position on each side, while the leading shoes are installed in the front position on each side. If the shoe positions are reversed, the braking performance will be very grabby and prone to wheel lockup.
•Brake linings contaminated with brake fluid or gear oil.
•Incorrect shoe size or type.
•Loose or broken brake lining.
•Brake shoe adjustment too tight or too loose.
•Weak or broken return springs.
What to Check For:
•Look for the following common mistakes:
•Leading and trailing shoes are reversed.
•Both leading shoes are installed on one side while both trailing shoes are installed on the other side.
•Inspect the wheel cylinders for signs of leakage or sticking, replace or rebuild as necessary.
•Inspect the axle seals for signs of leakage. Gear oil leaking from the axle seal will usually be indicated by a spray pattern starting at the center of the drum and spreading outwards. If any signs of gear oil leakage are found, the shoes and seal should be replaced.
NOTE: Leakage at the axle seals is often caused by worn wheel bearings. If the vehicle has high mileage, it is recommended to replace the wheel bearing along with the axle seal.
•Inspect all brake springs and replace if any appear worn or distorted.
•Inspect shoes for correct fit with drum and backing-plate.
•Inspect brake lining for cracks, missing pieces or poor bonding to shoes.
•Check for proper adjustment, readjust as necessary.
