Tinman22 Posted October 15, 2022 Share Posted October 15, 2022 I've noticed the front drive axle is wearing brakes far faster than the rears. The rear chamber's are 24's? and the fronts are 30's. I have not seen this before. Assuming everything is working properly uneven wear makes sense. Bigger chamber more preassure. Truck is an 85' R688st. Is this right or have they been changed out incorrectly? If so which ones are wrong? Any thoughts? Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted October 15, 2022 Share Posted October 15, 2022 I would suspect they are incorrect. You would want equal braking pressure all around. The fronts are picking up the slack for the rears. 1 Quote Link to comment Share on other sites More sharing options...
Hans Remmers Posted October 15, 2022 Share Posted October 15, 2022 I would switch to all 30's if you have sufficient clearance around the chambers to do so. 30's are more readily available and likely cheaper. I remember in the 90's 38 rears on Mack/Hendrickson air leaf didn't have clearance for 30's. Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted October 15, 2022 Share Posted October 15, 2022 Well all I can say is.... replace the fronts with 24's, put 30/30 maxis on the rear.. As far as auto slack adjusters go, on the rears you may have a hard time finding ones that will mount to the s-cam. I tried to do this to 2/ 86 RW's, and I couldnt find a parts guy that new how to cross the Mack numbers. the left and right are different. and the s-cam on the left side was too short ( by about 1/4") to allow a modern ambidextrous slack adjuster to be secured with the snap ring at the slack adjuster.. this being said,,, I am not 'all knowing' there may be a good auto slack out there that works. We just put the manual slacks back on and maintain proper adjustment,,,, just like in the olden days.. and with that being said... you can adjust your brakes to perform the way you need them too, and be safe and legal... I welcome all opinions and corrections about my posting... jojo 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted October 15, 2022 Share Posted October 15, 2022 My last post is if you have Mack rears... sorry I forgot to add that... Quote Link to comment Share on other sites More sharing options...
Tinman22 Posted October 15, 2022 Author Share Posted October 15, 2022 (edited) I don't think 30's will clear the rear. I'll go see what I can find tomorrow. Maybe I can find some 24's with springs for the front. They are mack axles. clearance is pretty tight . Something needs to be done for sure. as heavy as I usually am and the fronts carrying most of the braking its causing some tire wear issues also. Edited October 15, 2022 by Tinman22 hit post by accident. Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted October 15, 2022 Share Posted October 15, 2022 i would be surprised if they dont fit.. the rear.. its an 85 'R' model... Hell, I may learn something important here if i'm wrong.. with respect, Jojo Quote Link to comment Share on other sites More sharing options...
Tinman22 Posted October 15, 2022 Author Share Posted October 15, 2022 I hope they do but i can't fit my finger between the 24 and the housing now. I'll go get some 30/30's in the morning and give it a shot. Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted October 15, 2022 Share Posted October 15, 2022 (edited) do you have an old 30/30 in the scrap bin that you can use for mock-up? you do know that you can loosen the clamp on the service chamber and rotate it to a position that fits??? Edited October 15, 2022 by Joey Mack more content needed Quote Link to comment Share on other sites More sharing options...
Tinman22 Posted October 15, 2022 Author Share Posted October 15, 2022 I would think so unless the kids have been raiding the scrap pile again. 1 Quote Link to comment Share on other sites More sharing options...
Solution Joey Mack Posted October 15, 2022 Solution Share Posted October 15, 2022 I have installed many 30/30's on R models.. the push rod length fall's between 4 1/4" - 4 1/2" on the Mack rear's.. but don't take my measurement as creed,, do your measurement, the 30/30 is a close fit.. I hope a few others chime in with their knowledge, just so you get a full scope.. jojo Quote Link to comment Share on other sites More sharing options...
Popular Post Swishy Posted October 15, 2022 Popular Post Share Posted October 15, 2022 (edited) The most common garden variety of Paarking Spring brakes 24/24 24/30 30/30 those # represents how many cubic inches per pot B4 U cut the push rod <1> with spring brake on the floor conect shop air via a fitting into the park brake chamber <2> insert magic suking adaptor to keep push rod all the way in n nut up to hold sping compressed <3> release shop air ... <4> Cut push rod to desired length ( compare with other wheel chambers ) cya Edited October 15, 2022 by Swishy 3 Quote Link to comment Share on other sites More sharing options...
Tinman22 Posted October 15, 2022 Author Share Posted October 15, 2022 Thanks for the input. I'll sort it out tomorrow. I would prefer the 30's we will see. 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted October 15, 2022 Share Posted October 15, 2022 cya... Quote Link to comment Share on other sites More sharing options...
Tinman22 Posted October 15, 2022 Author Share Posted October 15, 2022 After going to three different parts stores I found a set of 30's with a small wedge built into the housing that clears the axle. My wife told me the story of her father replacing the front 26/26 chambers with 30/30's because they were cheaper some 15 ish years ago. So now I know why the front tires were cupping and the front brakes were worn out. Once again thanks for the help. 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted October 15, 2022 Share Posted October 15, 2022 Awesome....!!!!! Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted October 16, 2022 Share Posted October 16, 2022 On 10/14/2022 at 9:09 PM, Joey Mack said: do you have an old 30/30 in the scrap bin that you can use for mock-up? you do know that you can loosen the clamp on the service chamber and rotate it to a position that fits??? a pair of vise grips on the plunger output shaft helps to take off return spring pressure when loosening the SERVICE brake clamp. fortunately the new brake chambers only have one removable brake can clamp. the fun days of changing an emergency side diaphragm. 1 1 Quote Link to comment Share on other sites More sharing options...
mrsmackpaul Posted October 16, 2022 Share Posted October 16, 2022 Dunno what half of this mumbo jumbo is all about I have never had all this drama, 24s on the front, 30s on the rear Always smaller cans on the front than the rear so when your running bobtail and jump on the anchors real quick the back doesnt over take the front But as Swishman sez, watta eye no Paul 2 Quote Link to comment Share on other sites More sharing options...
Hayseed Posted October 16, 2022 Share Posted October 16, 2022 Swisho, If I'm Feeling lazy in a Hurry......😁 I just undo the Clamp on the service Chamber & swap everything from the diaphragm back over..!! Beats stuffing around measuring & Cutting Pushrods... 2 Quote "Be who you are and say what you feel...Because those that matter...don't mind...And those that mind....don't matter." - Link to comment Share on other sites More sharing options...
Tinman22 Posted October 16, 2022 Author Share Posted October 16, 2022 (edited) No drama just don't need accelerated tire/brake wear on one axle. mack calls for 26/26 front and 26's on the rear so i'm inclined to keep them matched and 30's are far cheaper and available. When I change service diaphragms or need to clock the fittings/clamps ,I cut a piece of tubing and slide it over the pushrod and put the clevis jam nut back on and tighten it to compress the spring. I run almost all two lane and usually 100-120 gross and with 30's on the front and 26's (not 24's) on the rear the front shoes needed replaced with less than 60k on them. Edited October 16, 2022 by Tinman22 Quote Link to comment Share on other sites More sharing options...
doubleclutchinweasel Posted October 17, 2022 Share Posted October 17, 2022 I think you are all over this. I have never seen bigger cans on the front axle. Seen same size a lot on dump trucks. Usually smaller fronts on tractors. Glad you got it sorted out. 1 Quote "Eagles may soar, but weasels don't get sucked into jet engines." Link to comment Share on other sites More sharing options...
mechohaulic Posted October 21, 2022 Share Posted October 21, 2022 am I missing something here!! when the statement says "30/30 on the front; im guessing "on the front refers to front drive axle. 24 go on front STEER AXLE ==OK. so this unit is getting 26/26 rear DRIVE and 30/30 front drive ?? Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted October 21, 2022 Share Posted October 21, 2022 bigger diameter cans 30/30 will displace more lbs pressure resulting in one axle working harder =quicker brake wear Quote Link to comment Share on other sites More sharing options...
mrsmackpaul Posted October 21, 2022 Share Posted October 21, 2022 Thinking there's a lot of confusion here The bigger the brake chamber the slower it responds as more air has to shift for the rod to travel the same distance However the trade off is the increase in diameter means the force is a lot greater Paul 1 Quote Link to comment Share on other sites More sharing options...
Tinman22 Posted October 22, 2022 Author Share Posted October 22, 2022 (edited) sorry for the confusion. the original post was "front drive axle is wearing brakes faster than the rears" After having them on for a week braking is much better. Now all drive axle brakes will lockup at the same time. Before the front axle would lockup very quickly when empty in the rain or off road. Edited October 22, 2022 by Tinman22 1 Quote Link to comment Share on other sites More sharing options...
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