94 Sonoma brake issues

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nojj

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Hello folks.

I have a trusty but high-mile 94 Sonoma, 4wd w/ 4.3 vortec V6
Brakes went recently,
pedal nearly to the floor with limited mechanical braking.
We limped it into the shop, and thus far we put in a MBC (bench bled)
replaced both rear wheel cylinders, both back brake lines,
and both front calipers.
Also rebuilt the drum brakes on the back.
(ABS light went on a few months ago, but the brakes were still operational.)
Got plenty of pedal till the truck is started, then the pedal gets mushy.
I'm a backyard mechanic of sorts,
my son is a newer (but certified) mechanic working in a shop,
but we're both stymied.
Any help? Idees?
I'm noticing some reported ABS unit issues for that make/model
 
nojj said:
Hello folks.

I have a trusty but high-mile 94 Sonoma, 4wd w/ 4.3 vortec V6
Brakes went recently,
pedal nearly to the floor with limited mechanical braking.
We limped it into the shop, and thus far we put in a MBC (bench bled)
replaced both rear wheel cylinders, both back brake lines,
and both front calipers.
Also rebuilt the drum brakes on the back.
(ABS light went on a few months ago, but the brakes were still operational.)
Got plenty of pedal till the truck is started, then the pedal gets mushy.
I'm a backyard mechanic of sorts,
my son is a newer (but certified) mechanic working in a shop,
but we're both stymied.
Any help? Idees?
I'm noticing some reported ABS unit issues for that make/model
Click to expand...

It sounds like your ABS controller may have air in the system and will need to get bleed. Just keep bleeding your brake system as if its a conventional break system and once your brakes are firm. Find and empty parking lot and go about 6-10 MPH to activate the ABS and then re bleed the system again to get those pesky air our of the system.

Take note that the peddle will drop when the air works its way towards the conventional park if your brake system.

Keep us posted and stay safe;)

Molson02536
 
Thanks for the reply, Molson.
We'll give that one a shot, probably Sunday.
 
Quick update
We rebled everything, (even the ABS box) tightened the rear lines, but still mushy
even put hoses off of all the bleeders and into bottles to see if something was slow.

Then a curious thing was noticed:
When pushing on the pedal, the brake fluid was going INTO the rear reservoir
Determined that the issue is probably with the prop valve.
We tapped it with a mallet a couple times,
but only managed to make fluid go into the front reservoir.
 
nojj said:
Quick update
We rebled everything, (even the ABS box) tightened the rear lines, but still mushy
even put hoses off of all the bleeders and into bottles to see if something was slow.

Then a curious thing was noticed:
When pushing on the pedal, the brake fluid was going INTO the rear reservoir
Determined that the issue is probably with the prop valve.
We tapped it with a mallet a couple times,
but only managed to make fluid go into the front reservoir.
Click to expand...

Have your tried reseting your proportionate valve?

Behind a rubber cap on the Proportioning Valve there is a small plunger type that must be depressed to re-center a valve within the Proportioning Valve in order to get fluid flowing to the portion of the Brakes that experienced the leak/lack of pressure.

I hope this thread can help someone who is experiencing similar problems in the future with the knowledge of this valve within the Proportioning Valve that needed to be re-centered.

Keep us posted;)

Molson02536
 
there is a small plunger type that must be depressed to re-center a valve within the Proportioning Valve in order to get fluid flowing to the portion of the Brakes that experienced the leak/lack of pressure.

Yup, we tried that after getting results when tapping the PV with a hammer.
Had to call my guitar player,
who's head mechanic at a (dare I say it?) KIA dealership.
He gave us the heads up on the proportioning valve.
Yeah, he drives a GM..... and a Harley.:D

Oh, and there was some air in the ABS unit: good call on that one
 
Last edited:
Here's the latest:
Put in a new prop valve yesterday
(Had to order on-line, not avail locally)

bled everything, lot of bubbles in the ABS

Still has mushy brakes.
We're running out of options, having replaced most any brake part that moves.......
 
nojj said:
Here's the latest:
Put in a new prop valve yesterday
(Had to order on-line, not avail locally)

bled everything, lot of bubbles in the ABS

Still has mushy brakes.
We're running out of options, having replaced most any brake part that moves.......
Click to expand...

Still have air in the ABS system, its a PITA to getting all the air out of the system and at worst, your going to have to get a brake service tool to cycle the abs valves.

If you need to cycle the valves, but do not have access to a service tool (or if the dealer is not willing to loan theirs) you COULD just "go driving and slam on the brakes a few times to make the ABS work" to purge the used fluid from the unit.

This is usually NOT the most efficient nor socially responsible solution, though it seems to work just fine. You still need to bleed the car a second time, but it saves you from procuring the service tool.

Keep us posted and good luck;)

Molson02536
 
Thanks Molson, I'll pass it on,
and call some of my mechanic buds to see if a service tool is available
Monday-Morning--Where-Is-My-Coffee-.jpg
 
Finally got the brakes done in the Sonoma.

Got the ABS code off, then we used a MODIS to purge the ABS unit.
Took a long time, evidently there was a lot of air in it.
 
I know a guy who's addicted to brake fluid. He says he can stop any time.
 
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