I'm at the point (78000 mi) where I'm gonna be replacing the brakes and rotors. I know that soulds like a lot of miles on original brakes, but the mechanic that does my safety inspection says they are just beginning to be at the point of needing replacement. I found a great step by step write upon rear brake pad and rotor replacement on another JK forum, however it was not that specific on bleeding the system. My question is this, if in the course of changing pads, I unscrew the fill cap on the master cylinder and compress the caliper pistons, but don't open the bleed screws or anything in the sytem other than the fill cap, will I have to bleed the brakes after I replace everything? The brake replacement seems easy enough, I just want to do it right. Also, I have heard from friends who have done brake jobs that vehicles with rear ABS require the caliper pistons to be rotated while they re being compressed. If anyone is familiar with this, please feel free to reply. Thanks in advance, Keith.

 

No you're not gonna have to bleed it if you did it that way as you will not be introducing air into the system. The problem is that with the new ABS systems, it's not advisable to compress the pistons that way. Crack the bleeder open when pushing the piston back and when you're close to the end, close the bleeder as to not let air in the calipers. If this is your first time doing brakes, make sure you don't hang the calipers by the line (this could collapse the brake lines interior). Support it with a bungee, or coat hanger.

 

About rotating the pistons as compressing them: this is usually for vehicles that don't have a drum style parking brake as ours do, and the parking brake is integrated into the rear calipers. On those, you need to rotate the piston while compressing it because that's how the ratcheting mechanism works.

On ours, you shouldn't need to rotate while compressing them.

 

Thanks thrashr and Alec. I just wanted to make sure. thrshr, just to be sure if I crack the bleeder valve while compressing, I will not introduce air into the system. I assume I crack the valave as not to push fluid back up into the system? Also, I got the tips on how to do the service off the project JK write up section. The only mentioned the bleeder valve if i were to actually need to bleed the system, not for compressing the piston. I just want to be sure as not to cause more damage.

 

that's exaclty correct, if your compressing the piston and crack the bleeder then close the bleeder just before you're done will prevent air from entering the system. Cracking the bleeder does 3 things. 1. Prevents too much drain back to the system as to not damage to the abs system. 2. Easier to compress the pistons and 3. You get to remove some old fluid and introduce new fluid in the system, a semi flush if you will. Good luck and if you have anymore q's let us know.

 

You can change your pads as discussed above, but make sure you pump the pedal a few times after your done, but before you move the vehicle. Don't be surprised when the brake pedal bottoms out during the first few times. This happens because the new pads are retracted and need to move out and seat against the rotor. Once the brake pedal feels like it is at its normal height, then your good to go.

If it were my Jeep, I would take this opportunity to bleed the fluid anyway. Brake fluid (non-silicone type) is hydrophilic (attracts water), and after 78K miles, it has become contaminated with some water. The water in the system will corrode the metallic parts. Bleeding the system will remove any water and help keep the other parts in good working order.

 

While it won't hurt to crack the bleed valve when compressing the caliper, there is no need to do it. Just make sure the reservoir doesn't spill over.
There is no danger of hurting the ABS components by moving fluid backwards when compressing the caliper.

As stated above, a complete flush is in order if it hasn't been recently done.