Ephemeral brake problem

Naturally I haven’t been able to reproduce this issue when I have the car around other people, and it’s so occasional I sometimes forget it exists (bad car owner, bad!).

When I start my 1993 GMC Sonoma in the early mornings or after it’s been parked a long time (at least six or seven hours), pressing the brake to start the car works fine. However, the first time I apply the brake to stop the car’s movement, it makes a horrific THUMP sound and jerks to a stop. It does stop–just very abruptly. I can sort of feel resistance from the pedal. After the first terrifying stop it stops roughly and suddenly the next few times I brake without any noise, then smoothes out fine–it never seems to last more than a minute or two at worst and seems to be exacerbated by braking while heading downhill, although that could be my imagination.

It doesn’t happen all the time and doesn’t seem to have anything to do with the temperature, although that again could be my imagination. It’s been cold as anything lately and it hasn’t done it in the past two weeks or so–sometimes it will do it three days in a row and sometimes enough time will pass that I forget it happens at all. It started maybe a month and a half ago and I have just been too broke to do anything about it. Money’s better, so here I am.

I was reading about similar issues but most people with morning brake problems relate a real hardness in the brakes which doesn’t seem as bad as the slight stiffness in mine, and none of them have mentioned any kind of associated noise, so I wanted to be sure.

Thanks in advance! I know about much about cars as I do about the economic details of Latvia but I will try to answer any questions accurately D:

I wonder if it relates to humidity and the amount of surface rust on the rotors/drums?

You might have a broken brake pad. Have the pads inspected (or just replaced if they are more than a couple of years old; you mentioned hills that require braking).

I have a 91 Sonoma that was having a similar problem. I would press the brake and it would make a similar noise and then grab. It did not seem to do it all the time and was more noticeable when I pressed the brakes hard.

I found that when I took the calipers apart the smooth guide end of the bolts, the two bolts you take out to split the calipers have a thread part near the allen bolt end and then a smooth rod part that guides the brakes, had built up dirt and some corrosion. I took a 3m sponge with the scrubber back some brake cleaner and scrubbed them down clean and then added some silicone grease. The grab and noise stopped.
Hope that made sense to you.

That is very typical of drum brakes to do that…at least with harder, gluey metalic linings. I used to chamfer the leading edge of the brake shoes and had pretty decent luck. Some shoes are chamferred when they’re made. The idea is to keep the shoe-to-drum interface less than 90 degrees…the shoes will grab when they’re like that much like a crappy wiper blade jumping across a windshield. Chamfers also help prevent front disc brake squeal. I’m not talaking about removing lots of material…I used the edge of a file. Clean the junk off with BrakeKleen or the like when you’re done.

Leave the truck parked overnight outside with your mechanic so he can drive and experience it.