WHAT VEHICLE? The '98 C1500 in your signature? WHAT BRAKE OPTION CODE? Kinda guessing JB5.
GM recommendation is to bleed an entire gallon. See attachment.
(I think this is excessive, but I'm not the one writing the procedure.) Remember, the GM procedure assumes that the master cylinder has no air to begin with. This is not always a good assumption.
HOW ARE YOU BLEEDING THE BRAKES? Pressure bleeder? Vacuum bleeding? Reverse-injection bleeding? Gravity bleeding? Two-man "pump-the-pedal-and-hold" bleeding?
Best guess is that the rear wheel cylinders are leaking air into the system. Wheel cylinders are good at keeping fluid in, but not-so-good about keeping air out.
The Safety Switch is activated when there's a pressure imbalance between the front brake hydraulic system, and the rear brake hydraulic system. Which happens when one or the other systems has air in it, or a failed seal somewhere including in the master cylinder.
Well, no external fluid leaks. You've obviously got air leaks into the system, and--maybe--internal fluid leak(s) past the cup(s) in the master cylinder.
More air due to starting the engine is new to me. I have no idea.
I would start with the rear wheel cylinders, AFTER making sure the rear brakes are properly adjusted. If these are the craptastic 254mm (10") leading-trailing shoe drums as are typical on 1500s, they're almost always way out of adjustment because nobody uses the park-brake; and that's what keeps 'em adjusted.
If that doesn't do it, you may need to either bench-bleed, or replace and bench-bleed the master cylinder.
I'm well-known for hating on the leading-trailing shoe drums, AND on the 8.5" ring gear 1500 axle assembly. You could scrap the leading/trailing rear drums AND the weakass axle by swapping in a same-gear-ratio 6-lug C2500 axle assembly, which would be the MUCH stronger 9.5" ring gear with 11.x Duo-Servo rear drums. Downside is that your 5-lug wheels won't fit the 6-lug axle shafts. You can use the 6-lug wheels from the donor vehicle, or get 5-lug axle shafts for the replacement axle. A conversion U-joint, differential gasket, and four U-bolts and nuts would finish that axle swap. You'd still need to assure the replacement axle's brakes and axle seals were in good condition, and you'd want to remove the differential cover to inspect the gears, and put fresh gear lube in it.
Buy a gallon of brake fluid, follow the instructions in the attachment.
I was following this procedure* already, thank you for confirming this. JB5 brakes, C1500, 1998 as signature indicates.
So that means that I had enlisted two of my closest buds to help me with the 2-man procedure. We did it in the order stated in the manual. We always get to the Pedal Feel and it's ok-ish to squishy, but when the engine is started, it is squishy and we can find more air to bleed.
I have not used a special tool at the 4WAL as at least one service manual says to use. I figured that this might be part of a problem, but not necessarily causing more air to get in. I've read on the forums to get the special tool in place to bleed better/faster. (
CAN ANYONE confirm if there is just one special tool needed, left/driver side of the Kelsey Hayes?)
As for the rear brakes, the sad/funny thing is that I am suspicious that not only do I have a wimpy 10-inch leading/trailing brake, but it would appear that someone possibly swapped in an old axle and/or brakes. I get this from the fact that the rear bleeder screws have SAE threads, which I've read means 1970s wheel cylinders. They were 3/8-24 and 1.3 inches long -- like Ford cylinders or really old Chevy. Weird to find this out after seeing how every parts counter would get us the metric ones with a different design.
Perhaps the Red Brake Warning Light is a key to my problem -- it seems to be an indicator of (1) air near that valve, so the bubbling causes it to trip all the time, or (2) the rear cylinder(s) leaks invisibly and makes the light turn on due to low pressure at the rear. I think (1) is more plausible, putting the culprit inside the 4WAL ABS controller or the master cylinder. But since so much air is in the lines after restarting the engine, it is easy to set this light. So we learned to bleed first and test for squishy pedal 2nd.
I'm ok-ish keeping this rearend and brakes, making it sort of like an S10 for heftiness. I have a Super Duty for when I need a big truck. I just need to get it running for light town use.
I might be stuck with telling my buddies that we need to keep bleeding this bad boy. I guess I could replace the two brake cylinders and buy a drum kit while I'm there ... my theory is that if there is air sucking in, it's one of the cylinders, master or slave I cannot tell.
Now, early on my buddy did a vacuum (one-man) bleed and got the old, trashy fluid cleared up and thought he got the air out. But GM says not to do that, and I told him thanks, but we need to keep trying the 2-man by-the-book. And I kept telling them to
Trust the Process. But all 3 of us are beyond hoping and trusting in it and would be
shocked if it finally bled out.
One side brake shoe is good and tight, while the driver side seems to be out-of-adjustment to me. None of us really know how to adjust it, though I figured we could guess while the drum is off and get the shoes a tad bit closer to the drum like the passenger side.
*Note: there had to be a slight modification to the GM procedure. The rear cannot be bled in the way stated.
The pedal does not go to the floor. So I would carefully pump a little (not stomping) and then bleed the rear as a 2-man procedure like old school and have them shut the valve when they saw that flow had mostly stopped. When bleeding the front, the pedal acts just like the PDF says, and I followed it to a tee with full stroke to the floor. If by saying this, you deem it bad, then please say so. Perhaps this means that my proportioning valve or master cylinder is messed up.
