ABS and ignition problems

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1998_K1500_Sub

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Haven't had a chance to pull them yet, but I realized last night they're almost certainly the larger pistons. When I swapped in the D369 pads, I had to bend the springs on the back of the pads outwards to keep them in the piston while I was assembling everything. As they came out of the box, they just flopped back out with no pressure against the sides of the piston at all.

Valid observation.


I also realized that if these springs didn't seat quite all the way, maybe the pad itself could be pushing the piston back? I'll check that once I get back in there.

Keep us posted, everyone learns from it
 

1998_K1500_Sub

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At this point I'm tempted to go back to a QTU master and the smaller piston original calipers, if it'll make this thing roadworthy again.

If my opinion is of any value, I would suggest you not turn back to the QTU master / LD brakes. IMHO their braking response is too dependent on:

- the assumption that the caliper supplier actually (re)built the calipers so that the LD function is properly enabled,

- the optimal interplay between the MC and caliper cylinders, e.g.,

-- the volume QTU fluid required by the caliper to bring the pads into play on the rotor,

-- the volume of QTU fluid delivered by MC's QTU piston, and

-- the MC's management of excess QTU fluid via the check valve and compensation port

- See https://www.gmt400.com/threads/gm-l...ssion-on-jalopyjournal-com.60055/post-1365261 and the dialog throughout that thread


I ****ed with that QTU/LD brake setup on my Suburban for much too long -- literally years, trying to figure out why it worked so poorly -- and now that it's gone I would never turn back.

My brakes work great with 80mm/3.15" diameter calipers and Raybestos EHT370H pads, new rotors and an NBS MC w/ stock vacuum booster (I recommend the JB7 MC now, but I used the NBS MC then).

I also have SS brake hoses all-around and 11" drums w/ oversized 1.1875" diameter cylinders, and those hoses and cylinders did make a remarkable contribution.

These are the brakes on my '98 K1500 Suburban which tipped the scales at 3020lb front, 3160lb rear.

$0.02
 
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Caman96

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I had a similar multi-purpose tool in my first car. It doubled as a hood prop and a button presser. The trunk wouldn't open unless you were pulling up on the lid while you hit the release button, and the button was on the driver's side door panel. With one hand pulling up on the trunk lid and the other jamming the button with the stick I could just barely get it.
Let’s not forget Gen. Chuck Yeager and his infamous broom handle when he was the first to break the speed of sound in the X-1. :893karatesmiley-thu
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cdone

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If my opinion is of any value, I would suggest you not turn back to the QTU master / LD brakes.
I certainly don't want to, but fact is my brakes now are significantly worse than before the swap, so going back (but with new MC and calipers) would at least give a reset to where I know how it "should" work and feel. I wanted a spare reservoir cap to turn into a pressure bleeder cap, and it was $20 vs $30 for a new QTU MC with cap, so I bought the latter and it's now sitting on the shelf. So, easy enough to swap back and forth for testing.

I have the larger wheel cylinders for the rears also, but I'm not going to change more things until the first couple things are figured out. Doesn't seem like switching to WCs that I know use more fluid will help my case right now.

The ABS is still a big question mark in my mind... even though I have the new unit in there and it seems to work, it's also tempting to rip that out and bypass it. I just wish it was easier to do that in a reversible manner (that is, to get adapters to take the GM fittings down to a standard size for the given tube size). If nothing else, just to rule out the ABS unit as a cause of the soft pedal.
 

1998_K1500_Sub

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I certainly don't want to, but fact is my brakes now are significantly worse than before the swap, so going back (but with new MC and calipers) would at least give a reset to where I know how it "should" work and feel. ...

"I feel your pain"

Everything you've said resonates with me.

My only remark at this time is: Replacing the system you have with the QTU/LD system is, in effect, replacing a simple system with one that is more complex.

Occam's Razor can be mis-applied here with relevance: The simpler system is more likely to perform as intended.


Afterthought: I haven't retreated through this thread to see if you've replaced your hoses and confirmed you have solid plumbing throughout the system. I remember you've fussed with the ABS. I came away believing you were diligent in your work.

I mentioned this earlier: When I added the SS hoses and larger rear cylinders, the brakes improved notably. I forgot to mention that I also replaced a ruptured rear hardline at the same time. In fact, the hardline ruptured first, and I took the opportunity to install the SS hoses and rear cylinders during the repair, in one work-event.

For all I know, prior to the rupture, the section of rear hardline was soft/thin and, while not yet leaking, swelled under pressure to the detriment of the brakes' performance (this is speculation on my part).

The punchline is that, unless you know everything is in primo condition, you may have a devil lurking in some hiding place.

Keep us informed!
 
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