AeroNautical Posted October 15, 2013 Share Posted October 15, 2013 I did a disc brake swap on my 8.25 recently. Bled the brakes 3 times, checked every connection I could for leaks, but no matter what, the brake pedal will go down about half way and stop, and I can't skid my tires no matter how hard I hit them. Coming from drums it seemed like they did alot better job at stopping. Am I missing something? The load sensor is a little higher than normal, I'd say about 60% 40% in favor of the rears. The pedal just seems too easy to push down, too. Is there a way to adjust discs? Link to comment Share on other sites More sharing options...
Eagle Posted October 15, 2013 Share Posted October 15, 2013 There is no adjustment for discs. Did you follow the factory procedure for bleeding the rear circuit? Link to comment Share on other sites More sharing options...
Keyav8r Posted October 15, 2013 Share Posted October 15, 2013 Someone (Akula?) mentioned having to loosen the connections at the proportioning valve to get the last bubbles out of the system. Link to comment Share on other sites More sharing options...
AeroNautical Posted October 16, 2013 Author Share Posted October 16, 2013 I read the valve had to be at about a 45 degree upward angle while bleeding, but other than that I bled it the same as I do the fronts. You say loosen the valve, any more info on that? Link to comment Share on other sites More sharing options...
Keyav8r Posted October 16, 2013 Share Posted October 16, 2013 My bad. I used the wrong words - Akula69 said in another thread that after bleeding the system five times (3 quarts of fluid), he had to crack the lines at the distribution block (below the master cylinder), not the rear proportioning valve to get the last bubbles out of the system. The thread is a couple of days (or more) old and I think it has WJ Booster in the title. Covered brake bleeding issues. There's another thread about brake bleeding in between your post and the one above. Link to comment Share on other sites More sharing options...
Keyav8r Posted October 16, 2013 Share Posted October 16, 2013 MJ Tech Forum - Oct. 3 - "WJ Booster and MC, Now No Brakes" Link to comment Share on other sites More sharing options...
Eagle Posted October 16, 2013 Share Posted October 16, 2013 No ... and no ... and no. You don't "crack" any lines anywhere. http://comancheclub.com/topic/40428-brake-bleeding/ To add to those instructions: If the brake warning light does NOT come on, you're not getting it right. The second line to the rear is a fail-safe line that sees brake fluid/pressure ONLY if the front circuit fails, and the shuttle valve that triggers the warning like slides off center to open the bypass port. No light ==> no bypass. Opening the front bleeder simulates a front brake failure. Link to comment Share on other sites More sharing options...
AeroNautical Posted October 16, 2013 Author Share Posted October 16, 2013 One thing confuses me in those instructions, how do you adjust the proportioning valve? From the looks of it, it has a few brake lines coming off of it and an electrical connector, but no "lever" that he mentioned. Link to comment Share on other sites More sharing options...
cruiser54 Posted October 16, 2013 Share Posted October 16, 2013 You don't adjust the prop valve. I believe they're referring to the valve on the rear diff with the rod hooked to it. Link to comment Share on other sites More sharing options...
sloride Posted October 16, 2013 Share Posted October 16, 2013 Have you considered doing away with the load sensing valve and going with an adjustable proportioning valve? When I converted my '88 to disc, that's what I did. Worked like a charm. I'd recommend getting the speed bleeders, a huge time/labor saver. Link to comment Share on other sites More sharing options...
Eagle Posted October 17, 2013 Share Posted October 17, 2013 One thing confuses me in those instructions, how do you adjust the proportioning valve? From the looks of it, it has a few brake lines coming off of it and an electrical connector, but no "lever" that he mentioned. You are looking at the front distribution block. That's also the proportioning valve in a Cherokee. It is NOT a proportioning valve in the Comanche. The proportioning valve is mounted to the frame, above the rear differential on the driver's side. It has an arm on a pivot, which is supposed to be (but often isn't, on old MJs) connected to a ball stud on the differential by a vertical rod. Link to comment Share on other sites More sharing options...
Keyav8r Posted October 17, 2013 Share Posted October 17, 2013 That's what I did a poor job of clarifying - which is the distribution block and which is the proportioning valve. Both of my trucks still have the rear proportioning valves and I plan to keep them. I installed the speed bleeders and they are a great help for a one man bleeding job. Just make sure the end of the bleed tube is submerged in fluid - just in case the speed bleeder malfunctions. I've gone around the 88 twice and have good brakes but not great brakes so I will do the FSM bleed procedure after I get all of the other issues resolved. Reference cracking the lines at the distribution block to get the last of the air out, I just repeated what Akula69 said and it apparently worked for him. My policy about advice from the forum is to read everything I can find on an issue and generally go with the consensus. If that doesn't work, I may follow a few of the rabbit trails and then test for safe function before taking the truck to the road. There are guys on the forum who have obviously been around the block a few more times than I, so I usually give their advice more weight than others. Operative word is usually. Sometimes I can convince myself I've found the silver bullet, but it most often turns out to be a thin coat of paint over the brass. Link to comment Share on other sites More sharing options...
AeroNautical Posted October 17, 2013 Author Share Posted October 17, 2013 No ... and no ... and no. You don't "crack" any lines anywhere. http://comancheclub.com/topic/40428-brake-bleeding/ To add to those instructions: If the brake warning light does NOT come on, you're not getting it right. The second line to the rear is a fail-safe line that sees brake fluid/pressure ONLY if the front circuit fails, and the shuttle valve that triggers the warning like slides off center to open the bypass port. No light ==> no bypass. Opening the front bleeder simulates a front brake failure. Alright, I've experimented with how open the pass front bleeder port should be, but the brake light will not turn on. Just to make sure, I put my ebrake on to see if the light even works, it does. Do I have to slam down the brakes or something? Word for word I'm doing everything it says in those instructions. Link to comment Share on other sites More sharing options...
Eagle Posted October 19, 2013 Share Posted October 19, 2013 You should not have to slam down on the brakes, but the front bleeder has to be OPEN. There is nothing to experiment -- you are trying to simulate a FAILURE of the front brakes, so you want the front bleeder wide open, so it dumps the fluid. Link to comment Share on other sites More sharing options...
AeroNautical Posted October 19, 2013 Author Share Posted October 19, 2013 If that's the case, then I'm startin to think there's a problem with my braking system. The light will not go on, and I'm getting nothing but clean fluid with no bubbles. That's why I asked if there was a special technique to simulate that brake failure. I'm gonna try a vacuum tomorrow, if I can't get that line to purge, I think I'm gonna have to get rid of the weight sensing valve. As it stands I can't even drive my DD truck because of this damn issue. Link to comment Share on other sites More sharing options...
Eagle Posted October 19, 2013 Share Posted October 19, 2013 If that's the case, then I'm startin to think there's a problem with my braking system. The light will not go on, and I'm getting nothing but clean fluid with no bubbles. That's why I asked if there was a special technique to simulate that brake failure. I'm gonna try a vacuum tomorrow, if I can't get that line to purge, I think I'm gonna have to get rid of the weight sensing valve. As it stands I can't even drive my DD truck because of this damn issue. Vacuum what? The second line to the rear is an emergency bypass line. If the shuttle valve in the front distribution block hasn't moved off center, the port to the second (bypass) rear circuit isn't open and you can't pressure bleed it, you can't vacuum bleed it, you just can't bleed it at all. Open a front bleeder all the way, put a tube from it into a can or bottle, then stomp on the brakes. There needs to be a pressure differential between the front and the rear for the shuttle valve to move.. If there's a lot of air in your rear circuit, you may not be generating enough pressure difference to move it. Link to comment Share on other sites More sharing options...
AeroNautical Posted October 22, 2013 Author Share Posted October 22, 2013 After giving up on trying to make the front brakes "fail", I removed the fail safe line from the equation, and used a coupler to connect the proportioning valve in the rear directly to the flex hose line. In doing so, I've got very weak brakes again. But, no matter what, the pedal is still very spongy, and takes a second to spring back up when I let off my foot. I'm starting to think my brake booster is bad, or there is air somewhere and it can't get out. This is a picture of my new rear caliper. The nipple is up, the hose is down, however I have it wrapping around the top of the axle. Would this allow air to get trapped in the line? Link to comment Share on other sites More sharing options...
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