Brake bleeding issues

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rtaylor93

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97 K1500 W/Kelsey-Hayes ABS- A few days ago the brake pedal went extra low and on came the brake light. Still able to stop but extremely low pedal. Fluid level was perfect. Bled brakes starting at right rear using 2 person method. No change. Replaced master cylinder - bench bled and bled brakes. No change. Took it to a shop and had a pressurized brake flush done, still no change. Found some article on bleeding the ABS module. Started with pushing the little nipple on the ABS to purge the fluid. No change. Drove down the road and locked up brakes to make ABS kick in. Tried bleeding after that. Nothing. Replaced rear soft hose(fronts are pretty new and there is no pulling issue) and now it gets weird

First time cracking the bleeder the fluid shoots out in a nice stream with a lot of air. 2nd time cracking the bleeder there's barely any pressure and the fluid just trickles out. Somewhere along the line I noticed that if I held the pedal, cracked the bleeder. closed the bleeder, let off the pedal, started the truck, mashed/let off the pedal, killed the truck, held the pedal and cracked the bleeder again, well then I got my normal pressure at the wheel and a lot of air. So I did this about 20 times. Still no change. What am I missing/overlooking??
 

df2x4

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Did the shop you took it to have a GM Tech 2 or comparable bi-directional scan tool to command the ABS module to stay open during bleeding? If not, that would be my next step.

Here's the procedure for '90-'95, it should be similar to '96+. Notice it calls for a gallon of fluid. My mechanic only needed a little over half a gallon to do my red '97 C1500.

https://www.gmt400.com/attachments/...rake_bleeding_procedure_ref_cards-pdf.238379/

And here's another thread on it that Hatzie created, complete with a video.

https://www.gmt400.com/threads/92-2002-kelsey-hayes-4-wheel-abs-power-bleed.44315/
 

rtaylor93

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I believe they didn't hook up a scan tool and just simply did a flush. I didn't try the bleeding procedure from that literature. I'll have to give it a go tomorrow. I no longer have access to a GM tech 2 so it looks like I have no choice but to take it to a shop then?
 

1997

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gotta wonder what caused the issue in the 1st place, since you didn't lose any fluid.
any ideas on that from the shop that bled the brakes?
wrong master cylinder?

my 97 did the same, 1st was the rear wheel cylinder, couldn't bleed brakes either, took to shop, they replaced the MC, 2nd was a brake line popped.
 

delta_p

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Normally the brake warning light will come on because there is fluid leak and the pressure differential switch spool in the combination valve will shuttle toward the side with low pressure and cause the light to come on. Many people say the spool only serves the function to activate the light, and has no ability to restrict flow towards the leaking side in order to maintain some braking pressure to the other side when there is fluid leak on one side. The sides being the front and to the rear brake system. I have always thought that it did do some restriction when there was a leak and would help prevent loss of fluid. The confusing part is you were able to build some pressure in the rear during bleeding and you are not losing fluid anywhere.

The ABS is an isolated system (unless its valves are leaking through) and you should be able to bleed the brakes without activation of the ABS. Since you did activate it though, it's probably a good idea to make sure that the ABS is good and bled now too.
 

rtaylor93

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Alright so the problem is fixed currently. This is a long one but I hate those guys that never come back and let people know what happened. Figured the "new" MC I bought was bad. So I took the MC off my 92 3500 and it made a slight difference. Bore size is larger so still weird. Went to verify adjustment of rear brakes. RR was good LR wasn't. Pulled the drum and found the spring had popped off and the adjuster was all the way in. I've had the truck 70k miles and these shoes looked damn near new. So fixed that and adjusted. E brake feels tight now. Bled brakes and pedal felt decent. Drove down the road and then the brake light came on and the pedal started getting low again. I went to town and grabbed another MC. Bled it and got a good pedal. Backed up and heard a pop and then lost the pedal. Assumed it was that spring again. Grabbed a hardware kit and pulled off the LR. Found everything still in place but way out adjustment now. Pulled out the grinder and ground off the lip on the drum. Adjusted real snug - did the same to the RR since I was there. Bled the brakes. This time using the method DF2X4 mentioned(without GM Tech 2). Depressed the pedal and opened nipple. I ran a pint through the RR and got bubbles every other time. Around the 50th bleed I finally received no air for 10 times straight. Finished all the other wheels and now I've got the firmest pedal I've ever had.
 

Schurkey

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Went to verify adjustment of rear brakes. RR was good LR wasn't. Pulled the drum and found the spring had popped off and the adjuster was all the way in. I've had the truck 70k miles and these shoes looked damn near new.
So the rear brakes haven't been working all along.

Reminds me of the crappy drum brakes on my '98 Monte Carlo. 40K miles, look like they were installed a month ago.

So fixed that and adjusted. E brake feels tight now.
You adjusted the service brakes, not the park brake...right?

Backed up and heard a pop and then lost the pedal. Assumed it was that spring again. Grabbed a hardware kit and pulled off the LR. Found everything still in place but way out adjustment now.
I'm kinda mystified why the rear brakes can go out-of-adjustment that fast.

Pulled out the grinder and ground off the lip on the drum.
Your drum(s) are junk. Replace it/them.

(How can the drums be worn-out if the shoes aren't worn? Are these not the original shoes? Were the drums shot when the previous owner installed shoes?)

Adjusted real snug - did the same to the RR since I was there. Bled the brakes. This time using the method DF2X4 mentioned(without GM Tech 2). Depressed the pedal and opened nipple. I ran a pint through the RR and got bubbles every other time. Around the 50th bleed I finally received no air for 10 times straight. Finished all the other wheels and now I've got the firmest pedal I've ever had.
Let's hope it stays that way after you replace the drum(s) and re-adjust.
 

Jeff Janssen

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"I'm kinda mystified why the rear brakes can go out-of-adjustment that fast."

I'm curious if somewhere along the way, somebody installed new brake linings but used the old unmachined drums (leaving them with a lip). In that case, when the rear brakes were put together and adjusted, the brakes would feel like they were adjusted up tight. However, the new linings were only riding on the lip of the drum. Once the shoes settled into place or wore a corresponding grove on the edge of the linings, they were way out of adjustment again and were effectively useless. (in your case that happened all at once with a "pop".

Once you ground the lip off your drums, proper adjustment was possible and a thorough bleeding brought your firm brakes back.

Re-using old drums without machining them can be done... but I always have to watch out for that lip. In my experience that lip makes proper adjustment almost impossible.
 

Schurkey

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Drums typically have a "machining limit" of .060" increase in diameter. (.030 deep all the way around) Therefore, the lip couldn't be more than .030", and likely less than that because the worn drum has to clean-up at .060" oversize or less. You're not allowed to machine them farther oversize, because they'll wear in use.

Once machined to .060-over, they can wear another .030" increase in diameter (.015 deep all the way around) for a total of .090-over which is the "discard diameter" cast into the drum when it's manufactured. Of course, "metric"-sized drums have the discard diameter in metric units; and I'm sure that some manufacturers don't allow even .090 total wear. As always, go by what's cast into the drum, which should agree with what's printed in the service manual.

Machining drums with a hand-held grinder is primitive. That's why they invented brake lathes.

Those drums are now junk.
 

Jeff Janssen

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With the cost of machine work out here (if you can even find somebody to machine a drum properly) and the cost of new drums... those drums were already "junk" :)
 
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