mattimuss Posted January 19, 2015 Report Share Posted January 19, 2015 Every time I put a wrench to this car I'm either left scratching my head or I learn something new...or both. Right now I am scratching my head. Can anyone tell me why the wheel cylinders on my '41 P12 have two different sized pistons? I have never seen this before and it lead to some very interesting disassembly issues. I am sure there is a very good reason, and one of you veterans are going to educate me on this. I was able to get all four apart and cleaned up. It sure went faster after I discovered that one piston is larger than the other and cannot be persuaded to slide through the smaller one's end...no matter how hard you try! Question: Do I need to have them re-sleeved with two different diameter pistons or can I have a single diameter sleeve inserted and everything will work? Ok, class is in session and I am paying attention... Quote Link to comment Share on other sites More sharing options...
deathbound Posted January 19, 2015 Report Share Posted January 19, 2015 They're stepped bore wheel cylinders, don't know the physics behind it though. I had mine re-sleeved a couple years ago with stainless steel, stepped bore. Quote Link to comment Share on other sites More sharing options...
Young Ed Posted January 19, 2015 Report Share Posted January 19, 2015 The basic idea is that the wheel cylinder pushes one shoe a little more to compensate for the one that brakes better due to the direction the drum is turning. Quote Link to comment Share on other sites More sharing options...
TodFitch Posted January 19, 2015 Report Share Posted January 19, 2015 The basic idea is that the wheel cylinder pushes one shoe a little more to compensate for the one that brakes better due to the direction the drum is turning. To further elaborate, the force exerted by a piston is proportional to the pressure in the cylinder and the surface area exposed to the pressure. By using two different sized pistons they are getting a different amount of force on the two shoes. On a Lockheed style drum brake system, as the drum rotates it has a tendency to pull out (self-energize) the "leading" shoe but not the other. By using step bore cylinders they can compensate for that and give equal braking on both shoes. An alternative way of dealing with that is to have two half cylinders and pivot one shoe at the top and the other at the bottom to make both shoes "leading" which they did on the car side for a number of year. Or you can just go to a Bendix setup where both shoes are "self-engized" by the drum which many other manufacturers did and Chrysler moved to in, I think, the early 1960s. Quote Link to comment Share on other sites More sharing options...
mattimuss Posted January 19, 2015 Author Report Share Posted January 19, 2015 Thanks for the replies and information. After reading them I will have the wheel cylinders resleeved the correct way... Quote Link to comment Share on other sites More sharing options...
mattimuss Posted August 22, 2015 Author Report Share Posted August 22, 2015 Hey All, I have been working on the brakes as time allows and am almost finished. I started putting everything back together yesterday afternoon when I ran into a little problem. I have had the front wheel cylinders relined by WPR and it looks like they did a nice job. I reassembled the front brakes, heel cams in neutral position and toe cams out to prevent the pistons and rubber seals from being pushed too far in. I learned the hard way that the wheel cylinders will leak if the rubber seals get pushed too far in. All was going well until I needed to adjust the toe cams to bring the shoes in far enough to get the drums on. Everything is fine with the passenger side, but the front [small bore] piston/seal of the driver side leaks every time I get the shoes adjusted enough to get the drum on. The back shoe of that side is adjusted all the way in. The front will stop leaking if I adjust the toe cam out. But each time I bring it in to get the drum on it starts leaking. Am I missing something? Has this happened to anyone before? If there a fix? As always, and ALL help is greatly appreciated! Quote Link to comment Share on other sites More sharing options...
P15-D24 Posted August 22, 2015 Report Share Posted August 22, 2015 First did you have the drums turned so they are round? Do the shoes fit the radius of the drums? If not you need to get the shoes turned. Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted August 22, 2015 Report Share Posted August 22, 2015 in getting the cylinder relined, did you use the stock pistons that were in the cylinders or were they REPLACED by a kit...Reason I ask is that the length of the rods to the shoes vary..the rods match the piston...some are shallow and some are deep and therefore the rod length difference..these cannot be co-mingled.. 1 Quote Link to comment Share on other sites More sharing options...
mattimuss Posted August 22, 2015 Author Report Share Posted August 22, 2015 Good morning and thanks for the responses. The drums have been turned and the shoes relined. I do not have access to anyone locally who can match the shoes to the drums. I am pretty sure White Post Restorations had to use new pistons as the original ones were disintegrated beyond use. PA, please forgive my ignorance, but please define the shoe "rods" as I have never heard the term before. Are you referring to the top end of the brake shoes that push against the cylinder pistons? If so, then I am pretty sure they are no longer matched. Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted August 22, 2015 Report Share Posted August 22, 2015 on the break down photo in the book these are called the brake shoe push rod..they are the rod that is split on one end to engulf the shoe and rounded on the other to match mater with the bore of the piston... Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted August 22, 2015 Report Share Posted August 22, 2015 (edited) These are the different pistons and push rods.... I use the short push rods with new replacement wheel cylinders and drums that have not been turned over .030". I too have had the same cup leakage happen so I end up checking/ swapping pistons or push rods to correct. Also the piston cup lips can be different lengths! A lot to consider. Generally the shorter push rods eliminate this problem. My info I posted here just not apply to his 1941 Plymouth!!!!! This above push rod info is for 1946 and up MoPar cars. Edited August 23, 2015 by Dodgeb4ya 1 Quote Link to comment Share on other sites More sharing options...
mattimuss Posted August 22, 2015 Author Report Share Posted August 22, 2015 Well now, aren't those interesting looking little devices...there were none on the car when I took the brakes apart. In truth, I have never seen those before. There were none on any of the wheels. This could explain a lot! I have attached a picture of what I found when I took the wheel cylinders off the car. More questions: 1. Where can I get them? 2. How do I match new ones to the pistons WPR used? 3. Won't using these push the pistons further into the cylinders? I am already have in trouble getting the brake pads in far enough to get the front drums on. In my novice mind putting these would cause the pistons to be pushed further into the cylinders which will definitely cause leakage. The back brakes are all back together without push rods. This might explain why I felt they were never going to adjust properly... I am so close to finally getting the old girl on the road, but I want to do it right. After all, my wife and daughter will be riding in her with me and I want to keep them safe. As always, ALL advice is greatly appreciated. Quote Link to comment Share on other sites More sharing options...
mattimuss Posted August 22, 2015 Author Report Share Posted August 22, 2015 on the break down photo in the book these are called the brake shoe push rod..they are the rod that is split on one end to engulf the shoe and rounded on the other to match mater with the bore of the piston... Hey PA, your reply got me thinking that maybe I had missed something. So I went and pulled out the trusty repair manual. The push rods are not pictured in my manual. Here are the pictures in my manual and they represent the brake setup on my '41. Now I am really confused... Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted August 22, 2015 Report Share Posted August 22, 2015 (edited) Sorry...my error for not going back to the first post and picking up on the earlier '41 model..as can be clearly seen by your pictures, the pistons are such that the shoe rest INSIDE the notch the parallel cast risers create ..have you verified that the shoes rest inside these properly.. Edited August 22, 2015 by Plymouthy Adams Quote Link to comment Share on other sites More sharing options...
mattimuss Posted August 23, 2015 Author Report Share Posted August 23, 2015 Hey PA, no worries. Thanks for helping. You guys had me scared that I had overlooked something that seemed pretty major. The shoes seem to be aligning properly, but I will pay closer attention when I go out in the morning. I was going to swap shoes from the passenger side to the driver side and see if that makes a difference. Speaking of brake shoes, and this may seem a dumb question, but does the brake shoe that is fully lined go on the front or rear of each wheel? My interpetation of the pictures in the manual seem to show the shoe with the full lining on the front of the wheel. This is the way I have reinstalled them. As always, thanks in advance for ALL help/assistance! Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted August 23, 2015 Report Share Posted August 23, 2015 hey...after this morning's chat...I want to see success comments in your next post....... Quote Link to comment Share on other sites More sharing options...
_shel_ny Posted August 23, 2015 Report Share Posted August 23, 2015 Speaking of brake shoes, and this may seem a dumb question, but does the brake shoe that is fully lined go on the front or rear of each wheel? My interpetation of the pictures in the manual seem to show the shoe with the full lining on the front of the wheel. This is the way I have reinstalled them. http://p15-d24.com/topic/39423-brake-shoes/ Quote Link to comment Share on other sites More sharing options...
mattimuss Posted August 23, 2015 Author Report Share Posted August 23, 2015 Hey PA and Shel, Ok, I was able to check a few things this morning after our chat PA. I did indeed have the heel cams in the correct position, arrows facing each other. And I did have the shoes on correctly too. Double bonus! I measured the thickness of each of the front shoes and found each was an 1/8" or more too thick. I pulled the shoes off and checked them against the drums, two are out of round enough to be easily seen by my old eyes. I think I may be on the right track! I would like to thank you PA for your advice on reading the manual at least three times before asking questions on this forum. I re-read the brake section after our chat and learned a lot of new info I swear wasn't there the last time I read it. I would add that one should clear their minds and seek their "Mopar Zen" prior to reading the manual! I know I have read that section multiple times, but I am sure that I read it while I was working on the brakes, frustrated and seeking an answer that seemed elusive at the time... Furthermore, I would like to apologize to each of you who I have frustrated with any questions that seemed dumb, or could have been easily answered by clearly reading the manual. It reminds me of a saying I learned in college. One of my instructors was an old, retired USAF Master Sergeant. Whenever anyone asked him a "dumb question" he would walk up to the white board and write in huge letters "RTFB". RTFB = Read The F'n Book Damn Master Sergeants! 1 Quote Link to comment Share on other sites More sharing options...
Dave72dt Posted August 24, 2015 Report Share Posted August 24, 2015 Reading and comprehending what you have read are two different things. Sometimes it takes more than one trip through the material. I don't have a service manual for my truck since it's so modified little of the manual would apply. I do have repair manuals for a number of other vehicles I have/had or am working on and find the aftermarket manuals often lacking in details or just really poor advice on procedures. I would expect the factory service manuals to be much more detailed. For example, I have the 5 speed 4x4 transmission out of an Asian vehicle that may have or have not, bad synchros, based on the owner's statements, (NOT a car guy, son in law or I wouldn't be doing it anyway) in it. The manual told me how to get it out of the vehicle and nothing on disassembly, inspection and repair. Pretty easy to take it apart the wrong way and break or damage something irreplaceable so I'm a bit apprehensive about diving into it. Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted August 24, 2015 Report Share Posted August 24, 2015 Dave...they expecting you to exchange for a rebuilt or run the risk of installing a junkyard dog.. Quote Link to comment Share on other sites More sharing options...
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