Another thing to consider is the drive shaft. When you brake the front of the truck tilts down and the back tilts up, which changes the angle of the driveshaft slightly. Has there been any work on the driveshaft? Does it have a carrier bearing? Those can get mis-aligned. Do you have a two part driveshaft, splined together? If so, the splines might be sticking, so make sure that joint is well lubed. There’s a special synthetic grease available I’ve heard can be used to solve sticking problems with those splined drive shafts. Haven’t tried it myself yet. But my Ford truck has that sort of driveshaft and sometimes it makes a noise when I brake, usually only noticeable as I come to a complete stop. I also hear it sometimes when I accelerate away from a stop. Any sensation coincident w/the noise I’d describe more as a vibration than a bounce though.
As mentioned above, leafspring problems can cause weird symptom like that sometimes. The leaf’s should be free to slide across each other as the geometry changes. Maybe some of yours are rusted stuck together.
It might be difficult to isolate the cause with the tire out of round problem remaining.
@George_San_Jose1
Single driveshaft no carrier in thus truck. New u joints since rear most was binding outside of a range of motion. I thought I had found the problem like you suggested as the pinion noses down maybe the u joint was entering this sticky area, but no luck.
Got a good friend letting me borrow 2 wheels with new tires. Hopefully that may isolate something
@anon86613489
The 11 inch appears like the brake assembly will float around better than this 10 inch. The 11 in has the adjuster at the bottom so nothing is held firm. On the 10 in, a peg at the bottom holds both shoes in position. So if the drum has deviations in roundness, the leading shoe Jas no where to relieve that motion when the shoe hits the drum. Not an ideal setup. I know drums rarely get out of round, but it is a possibility.
So the 10 inch rear drum is a Simplex brake design and the 11 inch is the more common Duo-Servo design? The simplex would seen to be more sensitive to drum out-of-round, as you suggest. The Duo-Servo floats so it is less sensitive to out-of-round.
When the shoes start to grab the drums they will rock a little bit(servo action).That could be felt like an out of round tire in the rear but its not.My little 99 Corolla with the rear drum set-up has this out of round tire feeling in the rear everytime I press on the brakes.Remember that the proportioning valve wants to equalize the braking so the rear wont lock up on you.You could have your drums resurfaced and adjusted and that can take care of the problem.
If I remember correctly, these trucks had a vibration that was virtually impossible to repair. My brain says there is a center bearing in the driveshaft - but I am unsure of that - and it’s that bearing that is the problem…
So I finally found this issue. After really spending too much time on it, it turned out to be a passenger side axle leak. I convinced myself I looked for this before, but in the dim light of the garage and being in a hurry the drum looked clean so I did not look too hard. When I really inspected this drum set, the passenger’s had a copper colored streak in the pad and I looked behind the axle hub to see it was wet. I pulled the axles out, replaced the seals, replaced the drums again (Oreilly made them up for no reason) and put on a fresh, uncontaminated pair of shoes. No vibration now.
There is some banging around once you come to a stop and then let off, but many have said it is spring bushing wear and this Simplex system binding up on the drum. Doesn’t happen on normal stops, just harsh braking so it will do as is.
Ah, so it was just some axle oil leaking onto the rear shoe surface and causing the brakes to misbehave? Makes sense. Good diagnosis there OP, and thanks for the follow-up.
fyi years ago my truck ( drums-all-around) started making weird noises during braking, and it turned out that was caused b/c the shoes weren’t properly sliding on the backing plate. A little brake lube between the backing plate and the shoe fixed that.
