Still air in brakes after bleeding for a week

[MountainDont]

Post a photo of the brakes with the drum removed. In-focus, nicely cropped, and resized as needed.

Is the adjuster at the bottom of the shoes? Or just under the wheel cylinder, above the axle shaft? If the adjuster is at the bottom, someone has changed the rear brakes, perhaps by changing the entire axle. The crappy JB5 rear brakes have the adjuster above the axle shaft.

Aftermarket wheel cylinders may not have the same bleeder-screw thread sizes as the OEM cylinders. As long as they fit the backing plate, have the right thread size and sealing taper for the brake tube, and have the same bore size and length, they should work OK.

But I am suspicious of the brake tubing where it screws into the cylinders. Mis-matched SAE--Metric threads there? Incorrect flare on the tubing to match the wheel cylinder?

Attached. I apologize that my brake photo is from above. I can't get a better photo for a bit.

Don't forget that there's two more reasons that the BRAKE light could be illuminated.

I bleed with key on, engine off, so I can watch for the red light. It happens when I push a little more than the system can tolerate during a bleed on the rear. One time, the light came on for a front brake bleed, but it was easy to reset. When the rear is full of air (after running the engine), it's much easier to set the red light.

I have been the pedal guy for almost all of the bleeds because my local Chevy mechanic explained that the guy at the pedal needs to be careful and slow to keep from setting the red light. As soon as a buddy of mine got in the truck, he was pretty forceful on the pedal, which goes against procedure and what the mechanic said.

Why? Something else wrong with the rear brakes?

While I have to touch the shoes, I figured that a new kit would refresh those old springs and adjusters that may be 25+ or 50 years old (if it's an old axle). Plus, the left one is out of adjustment and it should have adjusted itself.

Anyone see any issue with the rear differential? As I said, it could be a 1970s rear end. I've got a photo of the brake line tee, as well.

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The ^ brake cylinder from the 1970s? Or aftermarket? That is sucking in air maybe? This is also the side that isn't adjusted perfectly, although we tested it and it is braking ok.

 

[MountainDont]

What made you start bleeding the system? Did you just get this truck and it had squishy brakes from the beginning?

I ask because somebody could have put the wheel cylinder(s) in upside down. Make sure the bleeder is on top of the hose inlet.

The brakes were squishy when I bought it. I would need to pump the brakes to feel confident on a stop, like I'm driving a 1950s vehicle I suppose. The oldest vehicle I ever drove was a 1977 though. I'll look and make 100% sure, but I think they are installed ok. I also replaced the front calipers with remans for a few reasons, but crucially to make sure they were not a problem. Rock had them for extremely cheap. And that necessitated the bleed.

 

[Schurkey]

Looks like a plain ol' ordinary GMT400 rear axle, with the expected crappy 254mm Leading/Trailing shoe drum brakes.

IF (big IF) the axle has been swapped, it was swapped with another GMT400 1500-series ten-bolt axle.

You're perpetually bleeding air out of the rear wheel cylinders, so I suspect an air leak into the system at or near the rear cylinders. Could be mis-matched threads on the brake tube, perhaps a cracked flare, or a mis-matched flare on the tube vs. the flare seat on the cylinder. There's other possibilities, too.

Replacing the wheel cylinders, and inspecting the flare-nuts and flares would be high on my list.

 

[larry91gt]

Did you replace the front calipers? Did you put them on the wrong side? Bleed screw is on top of the calipers?

 

[phantom 309]

you mentioned the brakes always have been squishy, a wore out master will give that feeling,. unplug your ABS , see how it feels.

 

[JoeMakesUVOs]

My helpers have given up. They say that there is no way my brake system still has air after doing the procedure so many times.

We did 4 brakes in order, then the ABS Bleed with the cheapest scantool on Amazon. The Foxwell NT630 Plus. It really seems to do a bunch of bleeding. And we bled 3 quarts of fluid through this over the last 2 weeks.

But there is still actual air. Here is a tidbit that has us stumped:
1. We can get the air out of the lines, which mostly shows up in the rear. And then, if I start the truck for even 2 seconds, there is air in the lines at the rear adjacent to the bleeder valve! (We are wondering how it gets there so quick)
2. We keep setting the red Brake warning light. The process to clear it entails having us start the truck and slowly pressing the brake until the light goes out. I can feel that the combination valve/switch (the part that sets the light) is actuated when a pulse is fed back to the pedal.
3. We have found no leaks of any kind.

My conclusion is that we have not fully bled it ... but why do we keep seeing more air, especially when we start the truck? The pedal is super-squishy, but of course, we have actual air in the system still. Should I start replacing the rear brake cylinders and the master cylinders? (My theory is that the only way for air to suck in is at a master or slave cylinder. The booster is dry - no brake fluid inside it.)

We have bled at least 6 times all around, and ran the ABS bleed 4 times.

Under the K.I.S.S. formula, have you replaced the master cylinder? My thought only on this I reckon, but if you're repeatedly getting air in the (mostly rear) brakes, and since master cylinders are (usually) oriented that the rear brakes are fed fluid at the port closest to the booster/pedal the majority of the time, just maybe you have a seal leak, or something of that nature, going on at the master cylinder. Also, if you did any component replacement (wheel cylinders, brake lines, etc.) make sure that all your fittings are solid as you could have inadvertently cross-threaded something. Brakes are a "closed system", so if you're getting air in them of any kind, the best formula is start at the top (master cylinder) and work your way to the brake assembly furthest away from the master cylinder, and check everything thoroughly along the way. Having come across this issue more than once, to do a quick check on the master cylinder for leakage = unbolt it from the booster, and observe the pedal end of the M.C. for fluid leakage, and the inside of the booster as well, for fluid accumulation/residue (it should be "bone dry" normally btw, if it's wet, you might be needing a new booster soon).
Also a little P.S. - I've had a few instances of proportioning valves/blocks getting stuck and creating a situation similar to yours as well, and a lot of factory proportioning valves also are the point that the brake warning light uses for the switch to activate the light in the dash....so maybe there's an investigation there?
I myself, have had virtually zero troubles on my 2x GMT400 trucks brakewise, other than the "usual stuff" of worn components, and a annoying tendency for my '96 to pop rear brake wheel cylinders.
Good luck!

 

[JoeMakesUVOs]

Attached. I apologize that my brake photo is from above. I can't get a better photo for a bit.

I bleed with key on, engine off, so I can watch for the red light. It happens when I push a little more than the system can tolerate during a bleed on the rear. One time, the light came on for a front brake bleed, but it was easy to reset. When the rear is full of air (after running the engine), it's much easier to set the red light.

I have been the pedal guy for almost all of the bleeds because my local Chevy mechanic explained that the guy at the pedal needs to be careful and slow to keep from setting the red light. As soon as a buddy of mine got in the truck, he was pretty forceful on the pedal, which goes against procedure and what the mechanic said.

While I have to touch the shoes, I figured that a new kit would refresh those old springs and adjusters that may be 25+ or 50 years old (if it's an old axle). Plus, the left one is out of adjustment and it should have adjusted itself.

Anyone see any issue with the rear differential? As I said, it could be a 1970s rear end. I've got a photo of the brake line tee, as well.

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The ^ brake cylinder from the 1970s? Or aftermarket? That is sucking in air maybe? This is also the side that isn't adjusted perfectly, although we tested it and it is braking ok.

Um, looking at the drum shot after already posting to the original article.....
I see what appears to be a leaking wheel cylinder in this shot!!
Pry the rubber cap seal off of the wheel cylinder (a little screwdriver will work, and you don't have to take the brake apart either) and see if there's fluid inside of it, if there is, there's your problem most likely - replace the wheel cylinder (I recommend you replace BOTH of them at the same time, and get NEW ONES ONLY to install because rebuilding them, or buying rebuilt ones, is a guarantee that you'll be back soon for another round of "drum brake repair 101" plus a new set of brake shoes to go with it.

 

[tinfoil_hat]

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[JoeMakesUVOs]

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Thank you tinfoil hat! EXACTLY what I commented on, exactly illustrated with what I see.
I'm still too new to know how to play with picture editing (or that you even could do so) just yet.

 

[MountainDont]

I have replaced both wheel cylinders now. They seemed suspicious. I don't think they were working correctly.

Also, my driver side adjuster screw was frozen up on the threads, which is why the drum was always so easy to install on that side. I broke it free and greased it per Chilton when reinstalling.

I am not a drum brake mechanic for sure. It took me many, many hours to put the shoes back on and put the springs on correctly. The adjust spring, which the Chilton manual said to install last, was a real pain. It goes on the "back" of the shoe and the "back" of the adjuster, as it faces you. I am thrilled that I bought a hardware kit, but mad that I didn't have the adjuster parts in the kit (because I cheaped out) when I got to the driver side. There was a tiny tang on the clip in the middle of the adjuster screw that was broken, so the clip can spin. No idea if that makes a huge difference.

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Under the K.I.S.S. formula, have you replaced the master cylinder? My thought only on this I reckon, but if you're repeatedly getting air in the (mostly rear) brakes, and since master cylinders are (usually) oriented that the rear brakes are fed fluid at the port closest to the booster/pedal the majority of the time, just maybe you have a seal leak, or something of that nature, going on at the master cylinder. Also, if you did any component replacement (wheel

Well, to actually keep it simple, replacing the Master cylinder is not on the top of the list since I saw no definite evidence of it having failed. First is the bench bleed. Second, is that this particular vehicle seems to be difficult to bleed when the master cylinder or ABS module are involved. Third, this MC has a lower portion for the rear outlet and it is further away from the piston/pedal/booster if I understand you right. Its fluid compartment is separate from the front brakes.

It's going to be my last option to replace... but I am suspecting it could be bad as stated earlier. I'm still not sure what previous owner(s) replaced except the rear line appears new.

I am now waiting on the time to re-perform the bleed and see how everything goes with new slave cylinders. At least I am confident that the rear brakes will work now as good as the front. I found a cracked shoe lining. A leading shoe had a hairline crack at one a pair of rivets.