Back brakes lock before fronts

[Cuba]

I've also found under adjusted shoes can make them lock up when lightly applying the brakes too. So common with GM throughout the years.

As @highdesertrange said, make sure the shoes are properly oriented.

Could be anything... but watching post now to see what it turns out to be

 

[Keith Seymore]

Once you determine there is no issue with the brake system, you can consider adding an adjustable aftermarket proportioning valve. My K30 has the factory load valve, I had an 87 K20 and 87 C30 that also had them. I'm not sure these were installed before 1981. I had a C20 3+3 that didn't have it, but I never had a lock up issue.

This factory rear load valve was short lived. It was from about 84 to about 87ish in 3/4 and 1 tons only. I actually thought it was to 86, but you say your 87's had it too. GM recommends by passing that and not to use it.

The rear height sensing valve started with the 1982 model year.

They ran through 1988, on the R30 and V30, as well as the G30 models.

They were rendered obsolete with the introduction of RWAL on the GMT400 trucks.

K

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[HotRodPC]

Certainly never knew it came on that early of trucks. From all I've read it was a short lived feature, and there are GM TSB's stating to bypass it and delete it. Not sure when the TSB came out, but I have seen that TSB before. It's been posted here before and I didn't think it had anything to do with Rear Wheel ABS.

 

[CookieSoup]

ever figure this out? i have the same thing with my 3/4 suburban. the brakes feel just fine but every once in a while both rear with lock up super easy. i will be removing the rear height adjusting valve when i do SS lines in a few weeks just wanted to see if there was another area to chase

 

[Cuba]

ever figure this out? i have the same thing with my 3/4 suburban. the brakes feel just fine but every once in a while both rear with lock up super easy. i will be removing the rear height adjusting valve when i do SS lines in a few weeks just wanted to see if there was another area to chase

I usually have this issue when the rear shoes aren't adjusted properly, when they are too low. Especially on cold starts, this issue would be more pronounced. If this is the case, simply adjust the shoes to have a slight drag. Make sure to cycle the e brake in the process to get them to seat all the way in the drum when checking the "drag"

 

[CookieSoup]

gotcha she sat for a long time before i got my hands on the truck so ill run a few more miles before i try and fight the rear drums off.