About 3 1/2 years ago I bought a 97 Silverado in really nice original condition, 126,000 km. The truck drove perfect.
After about a year or so, a weird brake issue slowly showed up. It was very subtle but over time I was noticing the brake pedal took a bit more travel to apply the brakes. The pedal effort was normal, and the truck always stopped, including panic stops, you just had to push the pedal farther than what I would call normal. Not bad but not like the 97 Silverado I had sold to buy this one. No visible leaks, never have to add fluid. If I pressed the pedal, let up and pressed it again, the pedal effort was normal still, and the pedal height was normal. It acted exactly like a master cylinder that is going bad and has an internal leak (bypass). However, if I was parked, did the two pumps to get the pedal height up and then pressed the pedal hard with both feet and held it for 30-60 seconds, there was NO pedal fade. Usually a bad master fades with that test but what else could it be? I almost ordered a rebuilt master cylinder a couple of weeks ago because it was bothering me, especially if I was pulling a trailer.
For fun today, I went to our service manager at our (former) GM dealership. He's a 59 year old licensed tech but he's been the service manager now for 25 years and he has an amazing knack for diagnosing unusual issues. I explained the condition and the first thing he said totally blindsided me. He said "Check your back shoes. It sounds like they're out of adjustment, not tight enough against the drum." Inside I was saying to myself "huh?" but I know him too well to question him. I explained my master cylinder theory and he told me to check the back shoes first.
Tonight I pulled the back wheels. I pulled off the right side drum and the first thing I see is these drums operate on a similar principle to the old drums on my old Pontiacs, but the self adjuster is up above the axle and attaches differently. I start to spin the adjuster wheel to tighten the shoes and then I notice the entire adjuster mechanism is actually so loose it has fallen out of place! At this point I smiled and thought "that sucker did it again, he called this one right"! I put it all back in place, tightened it up, inspected it to make sure nothing was missing, broken or misaligned and went for a quick road test. About 10 stops and the pedal felt normal all 10 stops. After YEARS of driving it with brakes that worked ok but still didn't feel quite right, it's fixed!
I wanted to post this just in case someone has one of those frustrating brake experiences that they just can't solve.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars
I had the same problem with my '87 S10 Blazer. I was doing a check on the emergency brakes and when I took the drum off the drivers side found that the cylinder was leaking. Changed both out and new lines as both twisted off, installed every thing and set the brakes up but after bleeding I had the same feel that you had. Phoned my son and he said the same thing as you licenced tech. Sure enough that was it.
I have seen that before too.
Also, a lot of guys with GMT400s (88-98) complain of mushy brakes. A common fix is to put a later master on it, say a 2002. Suppose to help a lot. It's a bolt on, just need an adapter for one of the lines
