Need help with brakes plz!

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1998_K1500_Sub

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I was just Googling "low drag calipers" when I came across this bulletin from Allied Signal (Bendix) which might be relevant to the OP.


In the bulletin they state:

"Most loaded caliper remanufacturers do not
differentiate between Low-Drag and conventional,
listing just one model to cover many
years of applications."​


and

''Combining a conventional caliper on one
side with a Low-Drag caliper on the other can
lead to uneven braking, regardless of the type
of master cylinder on the vehicle."​

If somewhere along the line the OP's calipers were replaced and the replacements aren't correct (e.g., a mix of low-drag and no-drag) it may be a source of problems.

Maybe Schurkey or others can comment on whether this concern is relevant to the GMT400s.

Interesting info on low-drag calipers and MCs is given, here.

 
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Schurkey

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So far as GMT400 vehicles are concerned, all the vehicles with 254mm (10") rear drums have low-drag calipers. JN3 has the smallest pistons, JN/JB/JD5 has bigger pistons for more force on the pads.

JB/JD6 has the same front brakes as the "5" units, but with the larger rear drums--approximately 11 1/8" Duo-Servo design. So the "6" brakes also have low-drag calipers.

JB/JD7, 8, and JF9 do not have low-drag calipers. The JF9 doesn't use Delco "floating" calipers, they're the crappy "Ford-style" Bendix calipers that slide on a leaf spring and a keyed block.





Low-drag calipers are like everything mechanical. They can get "stiff" where the piston doesn't slide outward easily; or they no longer freely float on the caliper mounts (or both.) Left long enough they'll seize, but until then they're requiring more hydraulic pressure to move. The Quick Take-Up master cylinder has a pressure relief valve in the Quick Take-Up system. If the calipers require more pressure than the pressure relief valve is set for...you're going to have a crappy, low pedal because all the low-pressure "quick take-up fluid" that is supposed to be supplied to the caliper pistons, is instead wasted by blowing-off into the reservoir.

The fix for this isn't to scrap the QTU master cylinder. The fix would be to repair or replace the calipers that have gotten stiff over time.

As removed, the pistons look terrible. They cleaned-up nicely. Same with the caliper mounting hardware. This is one of the JB/JD5 low-drag calipers currently on my K1500.
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You can see where I've scraped-away the hardened crust on the square-cut seal. I also ran a pick around the seal groove in the caliper to remove any corrosion. In general, the iron calipers don't corrode too badly. Aluminum calipers are horrible for corroding in the seal groove.
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How freely should caliper pistons move? I've only turned-up the pressure on my pressure bleeder beyond ~20 psi one time. I generally pressure-bleed at 10--15 psi. But I was having some issues with bleeding the brakes on my '97 plow truck (JD7) after rebuilding one caliper and replacing the other. As usual, the caliper pistons were pushed "in" all the way. I bleed them with minimum air-volume in the caliper bore, then after bleeding, I pump the brakes to push the pistons out so the pads are properly adjusted.

But this one time, I decided to turn the pressure up on the bleeder. At 35 psi air pressure in the bleeder, the pistons moved out on their own.
 
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1998_K1500_Sub

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In the course of servicing my 1998 Suburban over the years I first replaced the front calipers (JB6 to JB6), likely with Raybestos remans (but would have the check). When those failed to produce an acceptable improvement, I replaced the master cylinder (with another JB6 MC). Still, I saw no acceptable improvement.

The issue I was trying to improve upon was that my JB6 brakes had this really odd behavior (especially in abrupt-apply situations... urgent braking) of providing resistance on the pedal before the brakes would actuate... and this drove me NUTS. It seemed that you could literally stand on the brake pedal and it would resist... there would be no brake action until the pedal inched it way down, resisting all the way, and then the brakes would engage.

Call it "lost motion with resistance".

Under normal, non-urgent braking the system just seemed like it was a bit slow to actuate (some pedal resistance, some pedal lost motion), but under abrupt-apply situations the damn pedal would resist vehemently, and effectively delay brake actuation. Quite simply, this gave me (how would Freud say it?) "brake anxiety", the feeling that when I needed them the most they would take the longest to engage.

So if anyone can explain why the stock JB6 braking system worked like this, for me anyway, I would love to hear the reason.

I earlier posted “the rest of the story”. After installing the JB7 calipers and NBS MC, the brakes no longer exhibited this "pedal resistance without attendant actuation" behavior that I tried to explain above. The brakes finally worked like I thought they should... little to no resistance to pedal motion prior to brake actuation, then firm pedal thereafter.
 
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Erik the Awful

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The owner before me put in new brake hoses, new master cylinder, new rear drums, new rotors and pads up front. brake goes almost all the way to the floor, and pulls to one side only when the brakes are depressed. he seemed to think it was the abs module.
In other words he threw those parts on there, didn't do a thorough job, and is blaming the ABS module.

Adjust the rear brakes and then bleed the brakes thoroughly, including bleeding the ABS. Your brakes are likely just fine.
 

Schurkey

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In the course of servicing my 1998 Suburban over the years I first replaced the front calipers (JB6 to JB6), likely with Raybestos remans (but would have the check). When those failed to produce an acceptable improvement, I replaced the master cylinder (with another JB6 MC). Still, I saw no acceptable improvement.

The issue I was trying to improve upon was that my JB6 brakes had this really odd behavior (especially in abrupt-apply situations... urgent braking) of providing resistance on the pedal before the brakes would actuate... and this drove me NUTS. It seemed that you could literally stand on the brake pedal and it would resist... there would be no brake action until the pedal inched it way down, resisting all the way, and then the brakes would engage.

Call it "lost motion with resistance".

Under normal, non-urgent braking the system just seemed like it was a bit slow to actuate, but under abrupt-apply situations the damn pedal would resist and, effectively, delay brake actuation. This led to (how would Freud say it?) "brake anxiety", the feeling that when I needed them the most they would take the longest to engage.

So if anyone can explain why the stock JB6 braking system worked like this, for me anyway, I would love to hear the reason.

I earlier posted “the rest of the story”. After installing the JB7 calipers and NBS MC, the brakes no longer exhibited this "pedal resistance without attendant actuation" behavior that I tried to explain above... and I was elated! It finally braked like it should!
I was leaning toward a faulty power booster, until you said that JB7 calipers and a bigger-bore master cylinder fixed it.

I have no idea otherwise.
 

1998_K1500_Sub

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I was leaning toward a faulty power booster, until you said that JB7 calipers and a bigger-bore master cylinder fixed it.

I have no idea otherwise.

I took it to a local dealer (known them for years) and took the service manager for a ride, then let him drive it. He thought they felt OK. That's when I got *real* frustrated and started throwing parts at it... which worked out in my favor, in this case.
 

Stringer

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I was able to get my hands on the brakes. First I took the truck into a damp grassy field and locked it up about 15 times or so real hard. Then i jacked up the front and removed both front wheels, everything does look new except the caliper is likely rebuilt.

I took the cover off the master cylinder (also new) and filled it with fresh DOT 3. I started bleeding from wheels farthest from MC to the closest. I left the MC cap off to bleed and brake fluid exploded under the hood when pedal was pumped. I wasn't prepared for that bc ive bled my gmt-800 and it does not do that, i can keep the cover off to top fluid off. So each proceeding topoff i had to remove and reinstall the MC cover.

I have a good bleeder bottle and was able to see some air bubbles come out at various times during bleeding. The very last bleed was front driver's and despite cracking open the bleeder and having some1 vigorously pump the brakes, there was not enough pressure to push hardly any fluid by, just enough for 2 or 3 inches of fluid to sit in the line and not empty vertically in the bottle. Truck pulls the the right when brake pedal depressed. I have the JB5's.

Any idea if this is the caliper or the brake controller?
 

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yes they all look new
 

1998_K1500_Sub

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Evidently new hoses (and calipers and pads), but the hoses look to possibly be installed improperly. AFAIK the hardline points up from the banjo bolt, and the hose passes up and then across the top of the upper control arm… going from memory on the 4x4s anyway.

Does either hose get pulled exceptionally tight when you turn the wheels hard left or right? A hose with internal damage may lead to the problem you’re seeing.

Regardless, confirm those hoses are routed properly. Somehow I'm wondering if the right-side hose isn't on the left and vice-versa.
 
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1998_K1500_Sub

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Those pads LOOK new at first blush but appear worn… the piston seems to stick out quite a bit, if those are "new" pads, and the inboard pad looks thin for being “new”.

Maybe it’s the perspective of the picture that’s confusing me.
 
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