Stumped - soft pedal

[Chris253]

This is what I've done so far -

New brake booster, new drums, new rear brake lines, new rear brake shoes and wheel cylinders, removed abs module and replaced with proportioning valve from a 94 k1500, new master cylinder - tried one from a newer truck and a direct replacement, no change in either. I've bled the system several times and never gets better. Rear brakes were adjusted recently but I was told that the brake cables were frozen so the mechanic disconnected them. I understand that the cables play a role in keeping the rears adjusted but how fast would they go out of adjustment?
I tried to reset the proportioning valve by pushing the button and bleeding again- not sure if I did that right or not, probably not because I can't find a definitive answer on how to do it. What else should I try? About to sell this thing if it's not right very soon

 

[delta_p]

On this link near bottom are two books on the brakes. The one on base brake systems has a section on bleeding with a combination valve...be good to look that over. That one also covers the vacuum brake booster and how to verify your getting brake boost. There is another book at the link on the drum brakes that covers the shoe adjustment details.

https://www.gmt400.com/threads/88-95-service-manuals.43575/

With everything you said, it sounding like it's the shoe adjustment but not sure your point of reference for the softness. My brakes are pretty spongy when I compare them my newer vehicles.

 

[evilunclegrimace]

This is what I've done so far -

New brake booster, new drums, new rear brake lines, new rear brake shoes and wheel cylinders, removed abs module and replaced with proportioning valve from a 94 k1500, new master cylinder - tried one from a newer truck and a direct replacement, no change in either. I've bled the system several times and never gets better. Rear brakes were adjusted recently but I was told that the brake cables were frozen so the mechanic disconnected them. I understand that the cables play a role in keeping the rears adjusted but how fast would they go out of adjustment?
I tried to reset the proportioning valve by pushing the button and bleeding again- not sure if I did that right or not, probably not because I can't find a definitive answer on how to do it. What else should I try? About to sell this thing if it's not right very soon

Spend the money and replace the E brake cables before you do any thing else. I am going to guess that you have 10" rear drums? If so you need the cables to adjust your brakes correctly. Until the brakes are correctly adjusted you will not be able to diagnose the brake system correctly.

 

[evilunclegrimace]

You never mentioned the front brake pads. I’m assuming you checked them? Also make sure the pins that the calipers slide on are lubed properly.

On the rear shoes, they won’t need adjustment that fast. In the old days it was standard to adjust them every oil change. But I’m also curious if the mechanic ever actually adjusted them? Tighten them until you can barely turn the wheel. Back off 30 notches. That’s it. You can also try a little tighter to compensate if you think they’re not auto-adjusting like they’re supposed to.

30 notches is WAY to far to back off the adjuster. These brakes are Leading/Trailing design and auto adjust using the E brake cables to adjust the shoes. The 11" and 13" brakes are of the Duo Servo design and are able to be manually adjusted if the E brake cables are removed.

 

[evilunclegrimace]

This is another good point. New shoes are always spongy. Need to burn em in. Several hard stops forward and back can help with this.

All that does is glaze the shoes/drums and ruin a good brake job. Here is the proper way to burnish a new set of brakes.

https://www.wagnerbrake.com/technical/technical-tips/break-in.html

 

[evilunclegrimace]

You must be registered for see images attach

From the 1988 GM service manual

I am also a retired GM truck service tech. The Service manual page that you posted is for Duo-Servo brakes, not leading trailing brakes. You need to go to page 5A2-12,figure 23 for the proper adjustment procedure.

 

[evilunclegrimace]

You must be registered for see images attach

From the 1988 GM service manual[/QUOTE

That is for 11"/13" foundation brake.

 

[evilunclegrimace]

You are correct the procedure is for the duo-servo. However the procedure on 5A-12 is for adjusting the parking brake, not the service brakes. On page 5A2-8, it says that adjustment of these brakes is automatic and occurs on any service brake application.

That is also how you get the adjustment for the service brakes, You measure the drum inside diameter and the shoe outside diameter and adjust the star wheel to gain the proper clearance between the drum and shoes. You cannot adjust the star wheel with the drum over the brakes as there is not a adjustment window in the backing plate.

 

[Supercharged111]

Hydroboost and HD GMT400 master fixed my shjtty pedal that I lived with for 10 years. The 1500 now stops better than the 3500, gotta find some aggressive shoes for the rear of that. I have Hawk LTS pads awaiting install up front.

 

[df2x4]

gotta find some aggressive shoes for the rear of that.

If you ever do please post them up, I spent about an hour Googling around and I couldn't find anything that resembled a "performance" drum brake shoe.