Brake pulsation and hop from rear

[sewlow]

Welcome to GMT400 crappy brake land!
These trucks can be a bear to bleed the brakes. The prob is usually air STILL in that POS ABS system!
Pressure bleeding seems to be the way to go. Much better results.
Dealerships have some kinda system to do it right. Computer diagnoses? Dunno. Heard of that, but never had it done.
Maybe someone with exp. there can chime in.
A GMT800 master will take out a lot of that pedal sponginess common to these trucks.

 

[east302]

Dealer uses a scanner to actuate the abs valves and bleed the system. That was supposedly done by the Chevy dealer.

I need to see if the rental scanners from autozone and the like are sophisticated enough to offer that bleeding feature.


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[Nathaniel2g]

Does your brake pedal get stiff or come back to normal if you pump it a few times? Did you bed/seat the brakes properly before really driving on them?

 

[east302]

With the engine off, I can pump the pedal two or three times and it's firm. Crank the engine and it sinks to the floor. I then cannot pump it to firm it up.

I *think* I bed the brakes right. I avoided complete stops until I got on an empty road, ran it up to around 40 and did several moderate stops down to about 10 mph.

I've since read that you can isolate or narrow down wheel vs master cylinder or abs problems by clamping the three brake lines and checking pedal pressure, but I don't have any line clamp-offs and am concerned about crushing the lines.


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[east302]

Update... bought an ABS assembly from a junkyard and that fixed the soggy brake pedal. The junkyard part was $75 and included the ABS computer (EBCM - Electronic brake control module), brake pressure modulator valve and the combination valve so I'm not sure just which component was causing the issue. Would guess it was one of the valves.

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I did have trouble getting fluid to the rear brakes after installing the part. I think that the unit had a closed valve preventing flow to the rear brake line. I took the truck out and slammed on the brakes a few times to activate the ABS which I think reset the valve positions to allow rear brake fluid to exit the unit. Alternately, a shop with a scan tool probably could have reset the ABS valves and accomplished the same thing.

When replacing the brake pressure modulator valve, the shop manual says to "Verify that the combination valve metering rod is depressed during bleeding." This is on the side of the combination valve under a black rubber cap. When the brake pedal is depressed, the pin extends. They make a tool to keep the pin from extending, so I think that this tool is installed and then the scan tool automated ABS bleeding is started. Since I don't have a scan tool, I winged it. With the brake pedal not pressed down, I pushed the pin in and a good bit of fluid came out. I then held the pin down at its normal position (kept it from extending) while the brake pedal was pumped and released several times. I'm not sure what it really accomplished but think that it may have allowed some air to escape the combination valve into the brake lines which was later removed at the bleeders.

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[east302]

Oh well, spoke too soon. The pedal is good for about two stops and then goes soft again.

Edit: new master cylinder fixed that. Fingers crossed.