C2500 brakes weak then grab hard all of a sudden

[zanzabar]

1989 Chevy C2500 pickup, front discs, rear drums. Bled brake fluid recently, front pads good, have not opened rear drums so don't know. Normal braking is fine, usually everything works great. But occasionally when I've had to do rather quick hard braking I have found that there is not much stopping power at first and then I really stand on it and the brakes grab all of a sudden and stop the truck with a jerk. I can't tell if only the front or rear that are soft and then grab hard, or if it's all four at once, or what. Seems like it can happen when cold or hot so I don't think that is a factor.

Do I still have some air in the system? Is there a proportioning valve that is stuck somewhere? Is the master cylinder starting to go? Not sure where to look, any advice welcome, thanks!

 

[Astro14]

Contaminated shoes could also cause this, a bit of oil/grease film that takes a few seconds to break through. Take a look under your rear drums.

 

[Nick1994]

Open up the drums and pull back the rubber boots on the wheel cylinders to check for leaks, most of the time the inside of the drum is still dry while the inside of the rubber boots are wet, can make the brakes weird

 

[FL_Rob]

Could be a scored master or a vacuum leak to the booster or a combination of old truck with high mileage components that just don't work like they used to. Start with pads/shoes,rubber brake hoses, wheel cylinders, master cyl and/or vac booster .

 

[artificialist]

I would bet on the rear drum brakes to have completely failed. It wouldn't surprise me if you had leaking brake wheel cylinders and axle oil seals.

At this time, I would highly recommend new drum brake springs, adjuster wheels, and anything else like that.

 

[Joe_Power]

Run it about 60 mph, and slam on the brakes - as hard as you can. Do that about 4 or 5 times. And when I say slam on the brakes - I mean stomp on it to the floor. I've done this on my 93. It works

 

[zanzabar]

Drove it today, fairly cold overnight and it did this for the first two stops and then it was mostly ok. I will go ahead and open up the rear drums to see what it looks like, thanks for the tips!

 

[zanzabar]

To all who said look inside the drums: 10 points. Both sides had shoe friction material falling off, and one side had a couple springs loose and leaky cylinder. Time for an overhaul!

 

[Eddie]

Sounds like way past time)

 

[zanzabar]

Yeah, it was. I'm trying to decide if these are the original pads and everything. Sure doesn't look like it's been opened anytime recently but they can't possibly be original can they? 243k miles?

 

[silvercivicsi]

Originally Posted By: zanzabar
Yeah, it was. I'm trying to decide if these are the original pads and everything. Sure doesn't look like it's been opened anytime recently but they can't possibly be original can they? 243k miles?


My sisters elantra is still going on the original rear drums, 213,000 miles last I heard of it. Although your car is quite a bit larger and older I wouldn't be TOO surprised.