BlueDove91 Posted November 22, 2015 Share Posted November 22, 2015 Hey all -- I've read so much praise here about the brake booster swap modification on the MJ, I just had to try it out. Super easy install...used a pulled 99 Grand Cherokee MC/BB on my '91 6 cyl 4.0 MJ, didn't have to fabricate the rod or anything..fit great. Now that I'm on the road and testing it out, I'd like to ask about a couple of things I've noticed: 1. I did some harder braking at semi-higher speeds in our residential neighborhood, and there is a "squeak" or whiny/airy noise that comes from what seems to be the brake booster when I brake hard at higher speeds. Any thoughts as to this? Did I get a bad/failing booster, perhaps? Everything works fine in regard to the booster otherwise. The vacuum hose is snug and in good condition. Master cylinder? Didn't see any leaking fluid around the booster, MC, line fittings, or anywhere nearby. The sound is not consistent in degree each time, nor does it happen when I brake under normal /gentler conditions. It's certainly an irritating sound when it occurs, if nothing else. 2. If, previous to this swap, my brake cylinder seals were seeping a little bit, could the stronger brake booster send enough force to damage the seals even worse, right off the bat? After driving and "hard braking" for about 10 minutes, I parked the truck and it smelled of burning rubber...a little smoke still steaming from behind the Rear driver-side wheel from all the hard braking I suppose (don't worry, this is not something I make a practice of). The rim and drum area looked shiny with fresh fluid. While the question of whether the cylinder needs to be replaced probably doesn't need to be asked, I guess I'm simply curious as to whether the stronger brake booster could "bust out" weakened brake cylinder seals? 3. My brakes seem spongier than they used to be before the swap. To be honest, I was expecting a nice tight, responsive braking feel. Instead, it feels soft...not awful, necessarily, but not quite what I was expecting based on all the other feedback I've read. I might even say "not quite as good" as it felt with the original single-diaphram setup. Acceptable, I guess..but nothing especially noteworthy. And if I apply pressure to the brake pedal when parked, it doesn't really firm up at any point but kind of slightly compresses until it is close to (but not on) the floor. Normal? I had a buddy help me a little bit with the swap. He bled the valves while I pumped the brake pedal. If I remember correctly though, he went in the order of: RL, RR, FR, FL. From what I've read in the forums, you have to go in the order of RR, RL, FR, FL. Is that correct? Why is that the case? So based on the info I've given you, does anybody have any advice or experience with any of these "issues"? I'm not exactly a mechanic, just done some basic tinkering on my own stuff now and again, so a lot of this stuff I'm still navigating for the first time. Thanks for making it through this essay, and for any thoughts you might have to offer. Link to comment Share on other sites More sharing options...
shelbyluvv Posted November 22, 2015 Share Posted November 22, 2015 1. It could be a booster going out. Keep an eye on it and see if it gets worse. 2. The stronger pressures could have popped the wheel cylinder. I think it was just because you panic stopped it and that caused it. Replace both rear wheel cylinders. 3. You missed a step in the bleeding process. It goes as follows. RR, RL, FR, FL Open a front bleeder and press the pedal all the way to the floor. Re-bleed the rear brakes starting with the RR. Close the front bleeder and re-bleed RR,RL,FR,FL Link to comment Share on other sites More sharing options...
BlueDove91 Posted November 22, 2015 Author Share Posted November 22, 2015 Thanks. My buddy confessed that he had gotten quite a bit of brake fluid on the wheels when bleeding the lines, and especially messy on the one I was concerned about. So at least I know it's not spurting or anything. I will try the re-bleed at some point, although they are driveable right now so it might not be something I do immediately. What is the bleed order all about -- furthest from the MC up to closest? I'll keep my eye (ear) on the booster. It seems to be settling itself in a little more. Link to comment Share on other sites More sharing options...
SBpunk Posted November 23, 2015 Share Posted November 23, 2015 If you have the height sensing valve on the rear axle you need to bleed that as well. Link to comment Share on other sites More sharing options...
BlueDove91 Posted November 23, 2015 Author Share Posted November 23, 2015 If you have the height sensing valve on the rear axle you need to bleed that as well. Hmm, okay. I'm confident that didn't get done. Is that pretty common to find on most Comanches, or just on some? How easy is that to find? Link to comment Share on other sites More sharing options...
BlueDove91 Posted November 23, 2015 Author Share Posted November 23, 2015 And I guess I would also ask where that would fall in the valve bleeding order. Link to comment Share on other sites More sharing options...
BlueDove91 Posted November 23, 2015 Author Share Posted November 23, 2015 NVM...just did my own homework on that matter in the forums, which I should've done to begin with, really. I think Ive got enough info to getme by for now. Thanks. Link to comment Share on other sites More sharing options...
cruiser54 Posted November 23, 2015 Share Posted November 23, 2015 Did you notch the firewall seam so the booster could sit flush against the firewall, or do you need to on a 91? ? Link to comment Share on other sites More sharing options...
SBpunk Posted November 23, 2015 Share Posted November 23, 2015 Did you notch the firewall seam so the booster could sit flush against the firewall, or do you need to on a 91? ? Yeah its still too big to fit properly. I took a sledge and massaged it until it would sit flat. The HO you don't need to do anything if you have the white spacer thing from the booster. If not then measure the distance from the hole to the mounting surface and add washers until it matches. For bleeding the system its much easier if you have two people and a home made bleeder bottle. I used a gatoraid bottle (big ones) and some 1/4 tubing I think. Cut a hole in the top of the bottle so the tubbing would fit all the way down. Added some brake fluid to the bottle and used a smaller bit to drill out a breather. Worked great cept for the pissed of wife pressing the brake pedal a million times hahahaha. Link to comment Share on other sites More sharing options...
cruiser54 Posted November 24, 2015 Share Posted November 24, 2015 Did you notch the firewall seam so the booster could sit flush against the firewall, or do you need to on a 91? ? Yeah its still too big to fit properly. I took a sledge and massaged it until it would sit flat. The HO you don't need to do anything if you have the white spacer thing from the booster. If not then measure the distance from the hole to the mounting surface and add washers until it matches. For bleeding the system its much easier if you have two people and a home made bleeder bottle. I used a gatoraid bottle (big ones) and some 1/4 tubing I think. Cut a hole in the top of the bottle so the tubbing would fit all the way down. Added some brake fluid to the bottle and used a smaller bit to drill out a breather. Worked great cept for the pissed of wife pressing the brake pedal a million times hahahaha. I'm confused then. I put a 95 to 96 booster on my MJ with the spacer and it worked fine and I put a WJ booster on my XJ without a spacer and trimmed the lip on the firewall to get it to sit flush.... Link to comment Share on other sites More sharing options...
SBpunk Posted November 24, 2015 Share Posted November 24, 2015 I had to smack the lip on a 96 booster with the white spacer the other day. Made sure to move the canister breather and wires out of the way first. If you don't get that spacer though you need to use some washers to get it to line up properly on the 91+ Link to comment Share on other sites More sharing options...
cruiser54 Posted November 24, 2015 Share Posted November 24, 2015 Link to comment Share on other sites More sharing options...
SBpunk Posted November 24, 2015 Share Posted November 24, 2015 Yup same concept I just used a hammer Link to comment Share on other sites More sharing options...
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