Help Gauging Brake Shoe Wear

[1987 GMC Jimmy]

Okay, so I know what pads are and aren't supposed to look like, but what's the cutoff for shoes? To me, these look pretty worn, but maybe not completely to the nub. They look like this all the way around and no cracks. I was gonna run the race of trying to do all four wheels tomorrow when a machine shop opened that could turn the rotors and drums, but I'd like to put it off a few weeks if I could. Thanks.

 

[1987 GMC Jimmy]

I was testing the action and return on them, and a small disaster happened, I guess. I've seen people do the same test with the drum off, but I guess I wasn't so fortunate. The action on this wheel was okay, but it didn't return at all. Plus, it blew out a bunch of brake fluid from under the front seal on the wheel cylinder. The pistons appear even so I doubt one's stuck. Did I break this being dumb, or is it just time for new cylinders? I've got the shoes, and on the lack of return, can the springs just be cleaned and lubed?

 

[chengny]

I was testing the action and return on them, and a small disaster happened, I guess. I've seen people do the same test with the drum off, but I guess I wasn't so fortunate. The action on this wheel was okay, but it didn't return at all. Plus, it blew out a bunch of brake fluid from under the front seal on the wheel cylinder. Did I do something wrong, or is it by the grace of God that I picked this day to check my brakes out? I've got the shoes, I know I need two new cylinders, and I need to clean and lubricate the springs, huh?

Jesse - c'mon boy, you're better than this.

Not applying the brakes with the drums off (or the calipers lifted away from the rotors of disc brakes) is week-one stuff.

If the associated hydraulic circuit is pressurized (and with only return spring resistance to limit push rod travel) the pistons quickly exceed the point where they can maintain a seal against the ID of the wheel cylinders...brake fluid gushes out of the wheel cylinder.

Even across the full range of brake shoe wear, the effective stroke of the pistons remains nearly constant. This is due to the corrective action of the star adjusters. As the friction material wears, the star adjusters compensate by increasing in overall length - this essentially maintains the shoe to drum clearance to the same spec as when the shoes were new.

As for why your shoes failed to return to their un-applied positions (when the hydraulic pressure was released) - IDK.

1. It may have been simply that the pistons extended so far out that they got cocked in the cylinders.

2. Your return springs may be weak.

3. Another commonly overlooked problem - with brake shoes that fail to return to the "home" position - is that the contact pads on the backing plates are in poor condition (or lack lubrication)

Look closely at the condition of the backing plate and especially the contact pads. Although there is not any relative rotary motion between the brake shoes and the backing plate, there is a certain degree of linear motion that occurs - when the wheel cylinders force the shoes out against the drums. Even though the shoes are held tightly against the backing plates (by the mounting/hold down springs), the brakes are engaged by hydraulic pressure so they rarely (if ever) stick on the way out.

But when the brakes are released, the only force that returns them to the free wheel position is what is provided by the return springs. For that reason, the inboard edges of the shoes must be able to move freely across the 6 contact pads. If the pads are trashed they tend to prevent the shoes from fully releasing. The linings will then stay in contact with the drums causing premature wear. Or, as noted below, if the pads are modified - in an attempt to restore the smooth surface - this results in misalignment and grabby/noisy brakes:

 

[1987 GMC Jimmy]

Ahh, damn. I knew that was stupid on my part. I guess the saying of "believe none of what you hear and only half of what you see" applies here. I'll keep tinkering on it and see what I come up with.

 

[HotRodPC]

The good news is, those shoes look very typical of a proper working brake system in my opinion. They've been heated pretty good a time or 2 though. The fact you mention they look the same all the way around is good, no cracks so they even though they got hot a time or 2, it wasn't to drastic. They obviously never got a drum off inspection with all that brake dust allowed to accumulate. If those were bonded shoes I'd say they're 60% worn. Because they're Riveted shoes, I'd say they're 75%-80% worn. Yes, they're good enough to be put off for a few weeks if need be.

Rather than try to race or rush and do all 4 brakes at one time. I'd take your time and concentrate on 1 axle at a time. Do the rears, and do a thorough job. As cheap as brake parts are on 1/2 tons, don't even consider rebuilding the wheel cylinders. The cost difference is next to nothing for NEW wheel cylinders. Use Brake Clean and get all the nastiness off the backing plate. Use brake clean on the new shoes to make sure you clean off any grease from handling and installing the shoes. Have the drums turned. Make sure the adjusters are nice and clean and that the threads are clean and the adjuster moves freely. Lube the adjuster threads with White Lithium grease to keep them moving freely even if they get wet in the future. Lube those pivot points that Jerry is pointing out. Inspect your hardware for damage replace as needed or just replace the whole kit. If do them right, you shouldn't have any rear brake problems for several years. Because it so easy, it's never a bad idea to pull the drums off for an inspection when you rotate your tires or every other time you rotate your tires. When you do, spray brake clean and get all the brake dust residue cleaned off before reinstalling the drum. That will avoid all those black streaks you see in your current shoes. That's just brake dust getting smeared and imbedded into the shoe because it was allowed to build up. Brakes last many years, IF THEY ARE MAINTAINED. Of course towing and aggressive stop and go city driving means check them more often. Just like anything vehicle related, more abuse means maintenance more often. Like Trans Fluid and Filter change, normal driving 50,000 miles is fine, if you're towing a boat or toy hauler, then you change it at 30,000 or even sooner depending on the abuse you put it through. NO different with your brakes. Then of course, Disks don't require as much maintenance and quickly inspected and looked at every time you rotate your tires. So take your time, do the rears 1 weekend, then you can do the fronts , 2, 3 or 4 weekends later. Doing them correct and thorough is more important than getting all 4 wheels done at the same time. JMO