Brakes get spongey a few weeks after a good bleed.

[redfishsc]

This has happened three times in three years with this truck (99 C1500 Sub, vacuum brakes)

I can bleed the brakes and get a GREAT firm pedal. Over the next 3 weeks, they gradually go back to mushy. I have to push them halfway to the floor to get a firm pedal.

Also, I always take the truck on an old abandoned road covered in pine needles to engage ABS a few times and it never seems to suddenly change the feel of the pedal before/after, so I dont "think" I have air in the ABS.

Master cylinder is about a year old, replaced it last year thinking it was the culprit. Bench bled, also tilted it forward to "burp" it. Then I had them bled by professionals with scan tools that perform the Auto-bleed. Firm, lovely pedal at first, just like other times. Then, gradually fading to mushy.

Front calipers are 3 years old. Rear drum cylinders are 2 years old. None have leaks or soggy spots around the bleeders.

All pads and shoes are 1 month old.



Bleeders are also all new.

I'm kinda lost now. Could a caliper or cylinder be sucking in air without actually having a leak?

 

[east302]

Does adjusting the drums help?




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[454cid]

Front calipers are 3 years old. Rear drum cylinders are 2 years old. None have leaks or soggy spots around the bleeders.

Check better for leaks? I wouldn't expect leaks at the bleeders, I'd expect leaks at the seals. On the rear they would probably have to leak quite a bit before you'd see fluid on the outside. If it takes a couple of weeks you can't be losing much, and it might be that it's getting flung around and absorbed by dust and washed off by rain.

Are you seeing the level in the reservoir drop?

 

[Supercharged111]

What about a pinhole in a rusty line?

 

[454cid]

What about a pinhole in a rusty line?

If that happened next to the gas tank it would be easy to miss.

 

[redfishsc]

Fair points.

Even over the period of a year I haven't noticed any loss.


I'm going to try to see which leg of the system is the worst offender by bleeding them individually. I really think the air is getting in near the wheels (hopefully it's staying there and not floating up to the ABS unit)

 

[redfishsc]

Does adjusting the drums help?




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Drums have been adjusted out to the "smokes a wee bit" position many times lol.

 

[454cid]

Drums have been adjusted out to the "smokes a wee bit" position many times lol.

That makes me think you're expecting too much out of these brakes. My dad's 96 always felt mushy compared to mine with Hydroboost. If you're bleeding and then readjusting the rear brakes they may be too tight and then they're wearing quickly, and you're losing the feel you're after. How are the shoes doing for wear?

Or... are you sure the adjusters are working, and on the correct side of the truck?

 

[redfishsc]

I only adjust the rear brakes once a year or whenever because the adjusters do their job well enough. The shoes last years.

I don't think I am expecting too much because I own 2 1999 Suburbans and one has spectacular pedal feel. The only major difference is that one is a 4wd. Same brake system.

The 4wd one (which had nasty black fluid and obviously neglected) has a GLEEFULLY firm, high, brake pedal. It feels exactly as brakes on these trucks should. Has (ostensibly) the original ABS system and rear shoes that will need replacement in a year.


My 2wd (the one this post is about) has just as good of a high, firm pedal after bleeding (slow, steady foot pump method) for around 50 miles of stop-n-go traffic and then you start noticing that the pedal feels a bit softer and travels further.

Eventually it gets to where it doesn't firm up until halfway down and even then you can force it nearly to the floor (about 3 months after a brake bleed) .

Meanwhile the K1500 brakes don't change over time....not like this.

Rear shoes are definitely adjusted tight, and the star adjuster is cleaned and lubed with caliper glide.

 

[JackE]

I'm kinda lost now. Could a caliper or cylinder be sucking in air without actually having a leak?

Yes. Here is what I would do: With the brake pedal at it's mushiest, clamp off the rubber hose at each caliper and the rear axle. Is the brake pedal still mushy? If so, the problem isn't calipers, or wheel cylinders. At this point I would suspect the ABS module where the brake lines run into and out of. If the pedal is hard as a rock, take the clamps off the hoses individually, leaving the other 2 in place. When the pedal gets mushy again, that will be the culprit in the system.