This thread is to discuss the whys and hows of eliminating the pins that reside inside the disc brake cylinders and hence disable the compensation circuit Jaguar and Dunlop felt necessary in their design.
This thread is linked to another thread discussing the IRS.
Allow me to explain. I am not passionate about the pins and their compensating circuit; I just wanted to understand them. At the end of the day, it makes good sense to do “what works” IMO. So I would take the advice and recommendation of people far more knowledgeable, far more experienced, of this subject and the E-type than I ever will be. That said, these pins are captured by the mechanism inside the piston. The pins and the compensating circuit were intended to limit the amount of return the pads are allowed as they wear. This was thought to be necessary so the pistons didn’t retract fully into the cylinders thus creating a situation where the brake pedal had to be pumped in order to get the pistons close to the brake discs because they had retracted fully into the cylinders. It turns out, that this was an unnecessary worry on Dunlop’s part. Millions of disc brake manufactured since those early days have proven the pistons move away sufficiently to prevent wear and overheating and yet do not, in fact, retract fully into their cylinders. Hence the mechanism is superfluous. I believe that even though I did not remove the pins from my front brakes and they work just fine.
However, having to a problem with the front brakes is easy. Doing so with the rear brakes is not. My fear is the compensation circuitry may be contaminated or otherwise compromised and that may cause the pistons to not retract sufficiently to prevent undue wear and overheating. I see no way to verify the circuit is clean and functional before installing the entire IRS. So, can I safely cut the pins, eliminate the compensation circuitry and end my worry? Where/how should I cut the pins?
I’ll start the new thread.
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This is quite different from what I understand. I’m away from home so I can’t look, but doesn’t the manual specifically say the pin and the mechanism inside the piston is to retract the piston a short distance from the disc when the pressure is released? Certainly when you are disassembling the cylinder it is much easier to push the piston onto the pin and into the cylinder than it is to remove it.
Modern pistons use the seal to twist sort of and retract the piston after application, and additionally use a bit of run out on the disc to push the piston back.
Yes it does Terry. That is the compensation circuit. It develops a “feel” for where the pressure gets high - the point of disc engagement. It then let’s some of that pressure off from holding the pin so it can slip back slightly, while simultaneously letting the pressure in the pipe go to whatever the residual value is in the master cylinders. Hence the fluid goes to the static (residual) pressure but the piston has a clamp on the pin so the brake pad cannot retract all the way back to where it was before any pad wear took place. That “clamp” is why it is hard to pull the pin out, but relatively easy to push it in. It needs hydraulic pressure to release that “clamp”. My understanding at least.
These are the brake cylinders and their pistons with the pin. The 3rd one is the Girling cylinder/piston. All 4 are 1-3/4" diameter cylinders for the Series 1 Cars. The housings look good, but the cylinder bores are pitted and will have to be sleeved. I think the pistons are OK, but I worry what dirt may be inside the compensator circuit.
Scott, based on that statement:
Dunlop spent extensive engineering, research, testing, evaluating, money, etc. when they designed this brake. They then had much feedback from the field after the brake was installed and used in the real world. I do not believe they installed this solution just for fun. I would assume this design was a solution to an issue. I would say these are the knowledgeable people you are looking toward.
On the other hand, some here have anecdotal results that although I do not doubt have worked fine for them, that research is probably not quite as extensive as Dunlop’s. It does appear it is not absolutely necessary as reports with the design being removed have been okay. That is not unusual. Some solutions solve minor problems that may not be present for everyone. I cannot answer exactly why they did this. Future designs are not relevant, as this design uses a much different seal than what is currently used and that does retract the piston properly.
If the design is causing a problem, maybe it needs repaired, not removed???
But it is your choice.
Tom
The term compensation circuit is a new one to me. The bore has a fixed retractor pin that engages a self-adjusting retractor spring in the piston to pull the brake pad back from the face of the disc on release of pedal pressure. Self-adjusts to compensate for pad wear and design clearance is .008" to .010" to avoid dragging brakes. Nothing to do with preventing the piston retracting fully into the bore. Clean bore and piston will allow system to perform as designed and in my experience there is no reason to remove the pins. You could test the rear hydraulics and pad clearance before installing the IRS to confirm.
Good thoughts Tom, and not unlike my reasoning with the front discs. And I believe Jaguar and Dunlop had excellent engineers and technicians. My but is this. The front brakes are easily accessible. So if I had a dragging or overheating problem, I could get to it. The problem I worry about is there can be dirt in the tiny ports inside the compensator. How does one assure there is no dirt? And doing so without taking the compensator apart; I am not going to do that! Still, on the fronts, I went with the factory plan and on the 100 yards I have driven this car, they seem to work.
Then there’s the rears. I do not want a problem that requires the IRS to be dropped again. So I’m dithering. Keep the pins or remove the pins.
Robert, there is more inside the piston than just a simple spring. I’ll find the reference to a similar patent, by Lockheed I think. It is an interesting design.
Yep Paul. That is the question isn’t it. My guess is because Jaguar was early in the disc brake game, they designed a solution to a problem that, at the end of the day, wasn’t.
Since your cylinders need to be sleeved and rebuilt you might want to consider a set of brand new rear brake cylinders. Welsh has them on sale now for under $400 - that’s cheaper than the cost of sleeving & rebuilding your old cylinders so it’s worth considering plus peace of mind.
I guess so long as you can get that funny little spring washer in there that’s in good shape and does what supposed to, leave them in. If you can’t, chop it out.
Hi Gary. Thanks for the comment. What I see at Welsh is $571 for 4 cylinders. Maybe there is a sale I don’t see. But still, I seem to be hung up on the Dunlop originality. But you are right, buying the new would eliminate the concern over dirt inside the pistons.
I’ll give this more thought. So far, I haven’t done anything other than clean stuff.
I agree with Robert. The description that you talk about with a “compensation circuit” is not one I’ve ever heard of, and bears no resemblance to what I’ve read in the Jaguar workshop manual. The retractor pin is entirely mechanical in its operation, and unconnected with the hydraulic operation of the brake pad itself. Which manual does your description come from?
Paul, as I said in my previous post those millions of cars use a different design. As I know you know, they use a square o ring seal in a groove in the cylinder that allows the seal to retract the piston. The Dunlop uses more of a rubber washer attached to the piston. Different set up with different issues.
Tom
The piston internals need to be pretty much spotless for the retraction system to work, and if not then removing the pins would prevent binding but would rely on the rubber seals to deflect and then relax enough to retract the piston, which may well work. Certainly cheaper than new but YMMV. Suggest you build it up and test if you want to be sure pre-installation rather than post-installation…
Oh dear…I knew this subject would stir things up. This is going to be a long answer. My only intent is to understand these things and share the information. Let me confess: I did not understand how the pins and the little hole they slide into work. I found some threads that discussed the subject, including some on EtypeUK (that I now cannot find). But this one from @jagnweiner (whose car is just a few days earlier than mine) seemed relevant: S1 Brake Wheel Cylinder Rebuild - #10 by Dagenham
In Scott’s thread I gave an answer on Feb 21 that refers to a patent issued to Goodyear (not Lockheed as I misstated). That patent, issued in 1956, but based on an earlier one 5 decades earlier!) is so very close to what Jaguar did. There’s a picture I put up that showed the Goodyear design; that pic is a link that now doesn’t work. (Please don’t make me go find it!) In that patent, it is discussed how their “wear compensation” functions. They talk about “arms” but they are circular and look like the Dunlop “Bush” (the Retractor Bush) and they’re circular and surround the pin. They talk about a “cam” and that is a “wafer” and is equivalent to Dunlop’s “Washer”. Goodyear describes the mechanism as “hydraulic” because the brake fluid is providing the power to move or reset the washer by overcoming the spring pressure. If anyone wants to call this mechanical, there’ll be no argument from me.
Rather than me repeating what was said on Scott’s fairly short thread please take a look at it.
To answer comments directly:
Tom is correct IMO in that the Dunlop design is not the same as today’s disc brake systems in that the seal is a rather thick rubber washer and who knows how well that functions to precisely retract the piston so as to create a 0.008" - 0.010" clearance between the pads and the disc.
Having said that, there are many who have satisfactorily removed the pins and find their brakes work as well as ever.
Robert, when I said there is more in the compensator than a simple spring, it is because the “Retractor Unit” (Dunlop was not about to call it a “compensator” and infringe on Goodyear’s and other’s patents) utilizes the critical interplaying “bush” and “washer” to precisely create a minimal gap between pads and disc when the brake is off. The bush and washer are powered by the spring which, in turn, is powered by the hydraulic pressure.
David, the manual I refer to is the shop manual. A pic follows these answers.
Gary I appreciate your reference to the Welsh offering, but I’m stubborn and I want to use these dang Dunlop cylinders!
I agree Robert the piston internals have to be very clean and that is my worry. I also plan to assemble the IRS and test the brakes as you suggest.
Geoff, that early design is different from my setup. Mine has the “Retractor” peened into the piston and it is not easily removable as we all know. What I do not know (well I don’t know scheiss actually) is what the Bush looks like. Is it rubber? Brass? Steel? Whatever it is, its associated spring causes the Bush and washer to capture the pin (I think).
For convenience I’ll post the statement from Goodyear’s patent that describes the compensation event. Just think of the Bush as the Arms and the Washer as the Cam Ring. " This allows pin 13 to slide freely through gripping arms 27 until substantially all the deflection is taken out of the brake assembly Whereupon spring 34 moves arms 27 off of cam ring 30 as the fluid pressure behind collar 23 drops to thereby allow arms 27 to grip pin 13 whereupon the final movement of spring 34, pin 13 and piston 2 re-establishes brake clearance to prevent brake drag."
The whole point of Jaguar’s Retractor is to precisely maintain a 0.008" to 0.010" clearance between pads and disc as the pads wear. Disc brakes were used in industrial applications for a very long time and especially on aircraft since the 1930s or so.
My original worry was either dirt, or wear, or other malfunction would cause the Retractor Unit to not work properly and it would then be bothersome to drop the IRS. But I am happier having re-visited this subject because it does not appear the Retractor is as complicated as I though and therefore may not be so prone to accumulating dirt. My test of the rear brakes with the IRS out of the car will tell if it works…then. Only then. So, do I clip the pins or not.
Scott I’ve owned numerous E Types over the last 50 years, must have a least 400,000 miles in E Types, have pulled countless pistons apart and rebuilt them, and I’ve never had a failure of a retraction system. Some of those were really dirty and rusty inside but they all worked. Your fear is, I’m happy to say, misplaced. If you are concerned you should send the pistons to a rebuild specialist like White Post for example.
