GMT400 rear brakes and bounce

brakes
gmc
1500

#1

I have been chasing a bounce under braking on a 96 GMC standard cab short bed 1500. I have had the truck for 1 year and it is a high mileage truck. Bounce is in the seat and slightly noticeable in the brake pedal. Not consistent. Usually worse under light braking and around 40 MPH brake application speed.
Truck is stock suspension, everything (except knowing if front lower A arm bushings are good) is tight and has been inspected. I replaced the front rotors with Bosch parts and matching ceramic pads. I replaced the calipers, sliders and extension hoses due to right hand pull. Corrected that issue. I put on new rear drums (Oreilly house brand) and shoes (Oreilly house brand bonded) and new wheel cylinders due to finally having rivets touch drums in rear… I replaced the hydraulic rubber line to the rear axle due to mouse bites in the hose. Master cylinder also failed and I replaced that. Many times ahs fluid been flushed through the brakes system and appears clean. No lights on dash from ABS module.

I put on KYB GR2 rear shocks and Monroe heavy duty units for the front. This rides nice.

The tires are not round! I’ve had this problem before on a different vehicle and some of the issue could be from this. But the store I bought from claims they are good and will not make them up. When I put it on stands and run the truck in 1st gear I can see what I consider a large deviation, over 1/8 inch.

Last weekend I changed the rear seal in the trans and found the rear most U joint bad/binding in only one set of caps. Thought I had the culprit. I replace front and rear u joints with no change to the bounce…

So my question, has anyone had problems with bonded shoes on rear drums and fixed it with riveted shoes. Only difference is a place for some material to gather instead of rolling all the way across the shoe face. On the 10 inch rear drum setup, the bottom of the shoes are fixed unlike the older setup or the 11 in drum set-up. The application of the brake will jut into the drum if the drum is not round too. So I think deviations back here in drum surface could be more felt than the axles with the adjuster floating around at the bottom.

I am planning on pulling the new drums off, roughly 8K since install, and have Oreilly check them for true, maybe have them resurfaced. I have new AC Delco shoes at home now that are riveted. I have my hopes high this will fix the issue, but want to know if others have done the same thing or found other ways to fix this.


#2

Return the two out-of-round rear tires to the tire store.

Tester


#3

Up and down, or side to side?
If side to side i would guess a bent axle.


#4

Do You mean, that in 1 st gear, engine at idle, the radius varies with 1,8 inch ?
Something is wrong. Are You sure it’s not 0,18 inch?.
1,8 should make the vehicle allmost undriveable above very low speed regardless of balancing unless You are speaking of a horizontal (in lack of better words) deviation.
Could You give us a liitle more precise description.
If I had a wheel with ,18 inch out of round, either the tire or the rim would end up in the bin, depending on which was the culprit.
Actually, I’ve never seen a wheel 1,8 inch out of round on a car used on the road.


#5

Here’s a hint.

Tester


#6

Sorry, the tires run true. No deviation in the rim that can be seen. The tread is about 0.125 inch out of round in an up and down pattern.

Tire store will not do anything for this and simply clams the truck and tires are fine. I’ve been in there multiple times and they are tired of dealing with me.

Just out of round would not cause intermittent bounce under braking though. Would it? It would be there every time if it was a consistent tire issue.


#7

You need to describe that a bit better… Does the rear axle feel like it’s bouncing up and down when you apply the brakes? Equally side to side? Or out of phase - i.e. tramp? Have had someone in a car beside you to observe if it actually does bounce up and down or tramp?

Or does the axle wind-up and bounce back and forth rotationally? Same suggestion, have someone watch.

The out of round tire would likely be felt all the time, not just braking. Have a fancy tire store or dealer put the rears tires on a “road force” machine and see how far out it is. Are they big diameter, wide off road tires? Is the truck lifted?

Does this truck have all wheel drive? 4 wheel drive or 2 wheel drive? Does it have a locking rear axle?


#8

Now, this is getting absurd. Now deviation is about 0.125 inch. Thats a HUGE change.
Let’s say it was 1,8 inch, it certainly could cause some strange behaviour from the affected wheel when braking - aside from all the other strange behaviours when not braking.

Edit.
I think Mustangman is on the trail as a keen dog. He’s a man worth listening (well, reading) to.


#9

@Asterik, The original post was simply a typo. I tried to put in 1/8 inch and put in 1.8. Sorry.

@Mustangman It feels like the seat is trying to be tugged from under your butt as you move forward under the braking. I don’t feel side to side movement. But for axle tramp, I’m not sure and will have to see if I can repeat enough to get someone else to view or drive while I look. It is very intermittent.

Out of round can be felt on new asphalt even as low as 20 MPH. I know these are out of round as I have had this before on other vehicles. My estimate may be too small of the tread deviation. But you can see the tread move up and down clearly.

My hope was to see if anyone else had experienced this on a GMT400 truck and actually fixed it. We have 3 including mine at work and all three exhibit some sort of bounce at 45. My truck is a 2WD, open carrier 3.42 with pretty much stock height tire. I did upsize from 235/75 15 to 255/70 15.


#10

Swap the front tires with rears.

If the bounce moves to the front, it’s the tires.

Tester


#11

Two weekends ago I rotated the tires. If anything it is slightly better than it was but did not go away. The two that are now on the front were replaced by the store after my last attempt but they only replaced 2. The 2 original purchase tires are at about 10K miles. The two newer tires are about 5K miles.


#12

I don’t know the setup on Your car. Do You have a long and a short brake shoe at each rear wheel?, Could it be that they are reversed? It’s possible on some cars. I don’t know what that would cause on Your car.


#13

The rear brakes to lock up.

Seen that one!

Tester


#14

Thanks Tester

I assume that i’m the one You responded to :smile:


#15

Does the ABS light work?


#16

That’s interesting! Sounds like the axle is winding up… check the forward sections of the leaf springs and the front spring eye bushing. If the front of the spring is no longer clamped - the strap that holds 3 of the 4 leaves together, that may be magnifying any out-of-round or brake pulsation problems. If those leaves are loose, the windup rate on the spring decreases. On a truck this old, they may have rusted away.

You still may have a brake pulsation and tire-out of round problem, though. I’ll keep thinking…


#17

Yes lights are still functional


#18

Not sure about where you guys live, but here very light rust is all that I have. No rush through . I’ll look at the spring pack retainers that is a great thought.


#19

@asterisk
This shoe set was really strange. All 4 shoes are same length surface area. The originals were also. I think that is part of this 10 in drum setup.

A lot of people on the Chevy truck forums change over to the 11 inch drum setup. Getting harder to find donors though.


#20

Ah, that ain’t gonna make life easier. Hope You’ll get rid of the problem soon.