[Crawdaddy]

As some of ya'll may have read, I'm still working on upgrading my Suburban to a 3/4 ton. This process included upgrading the rear axle to a 14-bolt out of a 1-ton truck. So, the brakes in the rear are a lot bigger. As a result, my stock brakes weren't doing the job. So, I've upgraded the master cylinder to a JB8 master cylinder. I left the 1/2 ton booster in the truck since it appears the same one as the 3/4 ton one. So, logic dictates that since the master cylinder is rated for 1-ton usage, it should be able to make the truck stop well....not.

Now, the question is, why isn't the truck stopping well and able to lock up its brakes? My first thought is that I think I accidentally ran the master cylinder dry while bleeding the brakes. The brake pedal felt great and rock hard after bleeding the rear brakes, but went to nothing as soon as I started bleeding the fronts. When driving, I can stand on the brake pedal with both feet, and while the truck leans forward a lot, the wheels will not lock up at all. Braking distance is also considerably longer than it should be. So, I'm planning on getting some more brake fluid and re-bleeding the brakes. Should I re-bleed the master cylinder before bleeding the brakes? While I'd really rather not have to take the brake lines loose again, if I have to, I have to.

Any suggestions where I should go to get my brakes braking nice and hard?

 

[frozenrotors]

Crawdaddy,

I would definitely re-bleed the master cylinder. And if you can, after doing that. Get the front end of the truck in the air so the front is higher than the rear. Then re-bleed the front calipers, tapping them with something to knock loose any air bubbles that may be trapped in the lines or inside of the caliper.

Make sure you don't have any leaks any where in the lines, around the caliper seals etc. Also worth noting. Since the rear brakes are now larger, I'm guessing the calipers and rotors are larger as well. This means you'll have to move more brake fluid to the rear calipers or wheel cylinders as well.

Does this truck have ABS ? If it does, you sometimes have to cycle the ABS control module by hooking the truck up to a diagnostic scanner at a dealer. This allows all the air trapped in the ABS valve to escape. (This usually occurs after air has been introduced to the brake system)

Changing out the Master cylinder also changes the amount of fluid moved through the system, and may change how the brake fluid is distributed Front to Rear.

Hope this helps.

Keith

 

[Crawdaddy]

I will re-bleed the master cylinder then. I think the MC also might be on a slight rear lean in the truck, so I'll try to roll the truck up on ramps to get the MC level before bleeding the master cylinder. When moving up to the 3/4 ton parts, the rear wheel cylinders got bigger, but the fronts actually remained the same size, in fact, the same part number. The truck does have rear ABS, but it is the real old archaic rear abs. There isn't really an ABS valve, it's just an accumulator on the line for the rear brakes. About the only thing not new to the brake system now is the brake lines, proportioning valve, and brake booster. Everything else is brand new. The parts not changed are 1/2 ton parts, but it doesn't appear that the 3/4 or 1-ton replacements are any larger to handle more load.

 

[frozenrotors]

Now that you mention all of this, and the more I think about it. It sounds to me like yo still have air trapped in the system. The purpose of getting the front of the truck higher than the rear is; air will move or float to the highest point. So raising the front of the truck allows that air to accumulate around the bleeder valves (again tapping with a soft blow hammer or similar device will dislodge the air so it escapes through the bleeder valve on the caliper).
When you do the brake bleed, find someone to help you. Make sure the master cylinder never runs dry, and don't push the brake pedal all the way to the floor. This can ruin the seals in the M/C. Once you've pumped and bled the corner you're working on a few times. Completely close the bleeder valve and pump the brake pedal a few times until it's nice and firm. Then while putting pressure on the brake pedal, have someone slowly crack open the bleeder valve until the pedal starts to move. Once the pedal does down a few inches, hold the pedal while it's depressed and close the bleeder valve, and pump up the pedal again. And repeat the previous step.
This will force air to the top of the caliper and bleeder valve. And force the air to the highest point in the system. It's called POWER BLEEDING. Repeat that step on all four calipers and wheel cylinders until the pedal is nice and firm.
Top off the reservoir and drive it to see if it's working properly.

Hope that helps.

Keith

 

[Crawdaddy]

Hopefully the seals aren't already destroyed. I think we were going pretty much to the floor when bleeding the wheel cylinders. When we bled the front, the front of the truck was higher because I had taken the tires off the truck to get to the bleeders. I really hope I didn't destroy the seals on the MC, it's literally brand new.