Author Topic: W11x rear disc brakes dragging on '66 250SE Coupe  (Read 946 times)

Todd

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W11x rear disc brakes dragging on '66 250SE Coupe
« on: July 19, 2022, 00:24:07 »
Hi all,
Quick background:  Purchased the car in April 1992 (It was the seller's daily driver in San Francisco). Parked and stored car in 2009.  Prior to that, it was always very mechanically sound and reliable, other than the terrible rust in all the usual places.  But it was always "daily driver" reliable.  I wouldn't think twice about getting on the freeway and driving it umpteen hours, anywhere.  Recent work at that time included all 4 brake hoses, and replaced the cylinder head due to internal corrosion damage that was found when I removed it to have the guides replaced (found one of the exhaust guides broken, and the engine smoked badly on start up).

Fast forward to Jan. 2019:  I finally woke the car up and started returning it to the land of the living (I'm an idiot for having stored it).  I had to rebuild the fuel pump at the rear of the car, replace the seized heater blower fan, repair the badly rusted and seized sunroof motor, and free up the sunroof, and rebuild 3 of the brake calipers.  The right rear caliper had to be replaced, because I had incorrectly swapped on a W108 caliper with a smaller diameter piston, that I had never noticed was the wrong piston size (until I purchased a rebuild kit, and discovered it didn't fit that W108 caliper.  I also replaced the rear brake pads and rotors, bled the system, and confirmed the hoses were still good.  I did not replace the hoses.

The brakes have never been good on the car, and that W108 right rear caliper was clearly doing me no favors.  The brakes always pulled to one side and the rear always seemed to get hot, which is why I had replaced the original, sticking, right rear caliper at some point. (I had a W108 4.5 car that I took the rearend out of and installed in my coupe in order to lower the RPM's on the freeway)

I've driven the car very infrequently since getting it operational in 2019, because while the brakes were much "better" they still got hot, whether around town, or on the freeway, and would progressively drag worse and worse.  If left standing for a while, they would cool and free up, but then the process would repeat once driven again.   Well, I'm back to being able to/wanting to drive the car again, and want to tackle the issue, so here I am asking for help.  I don't want to just throw parts at it.  I took it out around town, got the brakes hot enough to be dragging, came home and shot the temps of each caliper with a temp gun.  The rears were both very hot at 125*F, verses the fronts at about 105*F, and car definitely feels like it stops "flat" like the rears are doing more work...verses that feeling of the car coming down on the front end.  I don't think the rear hoses are bad, but will remove and inspect them this week to confirm.  My question is: Will a failure of the (original) rear proportioning valve create this symptom?  Or could the brake master cylinder internal "check valves" be hanging up and holding pressure in that single rear line?  Anyone had this issue?  Apologies for just holding my hand out for a rescue, vs. tearing the car apart and doing my own diagnostic work.  Also, can you provide the contact info for the place that rebuilds these proportioning valves? 
Thanks in advance!
« Last Edit: July 21, 2022, 04:07:01 by austinado16 »
Todd

Cees Klumper

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #1 on: July 19, 2022, 05:44:42 »
I would replace all four brake hoses. I know you say you confirmed they are good, but they swell up on the inside, causing brakes to drag. They are relatively cheap and if they have not been replaced since the early nineties, combined with sitting for a long time with old brake fluid, I would bet that's a very likely cause of your issue.
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Vander

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #2 on: July 19, 2022, 10:37:19 »
I would replace all four brake hoses. I know you say you confirmed they are good, but they swell up on the inside, causing brakes to drag. They are relatively cheap and if they have not been replaced since the early nineties, combined with sitting for a long time with old brake fluid, I would bet that's a very likely cause of your issue.

I second what Cees says, replacing the hoses is actually where I would have started even before anything else. All 4 hoses should run about $80 total. We are talking about 30 year old rubber here
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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #3 on: July 19, 2022, 12:47:00 »
In order to determine whether it's the hoses that are the problem and presuming your brakes are indeed dragging such that with wheels off the ground you cannot turn them by hand, back off the bleed nipple on the caliper. If the wheel then rotates freely it is most likely that swollen hoses are trapping the pressure in the caliper.

Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #4 on: July 19, 2022, 14:41:42 »
Thanks guys!  I'm going to take a look at the hoses today to confirm length and whether, at least at the rear, which fittings they have (appears to be a version that has 1 male and and 1 female end, and a version that has 2 female ends).  I don't remember when I replaced them, but it could have been in the mid-90s. 

The reason I asked whether anyone has seen a master cylinder or a proportioning valve do this, is because both rears are equally effected.  Not that 2 hoses of the exact same age, installed at the same time, couldn't be partially clogged up inside and acting like check valves.  I'll report back in case this thread can help others.
Todd

Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #5 on: July 28, 2022, 17:42:33 »
Thought I'd offer a quick update:

Slight delay in official brake work, because I had the roof and trunk stripped and repainted last week.  Got sick of driving the car around looking so bad.  Got it back 3 days ago.

I've ordered all 4 brake hoses and those will be here in a few days (www.autohausaz.com).  I figured there's no sense in spending the time to pull wheels, remove an inspect hoses, wait for new hoses, etc. when I can just do the job right, once, the first time.

In the mean time, I wanted to get the car out on a remote stretch of road, get it up to speed, and do some hard, threshold braking, in order to see if the car would brake straight under full force (to the point that the front tires are just starting to make noise of almost starting to skid), and see if the car would pitch forward, indicating full braking force to the front calipers, AND not skid the rears.....or would the rears come on first, and skid, because the proportioning valve wasn't working.  I also wanted to see if the dragging brake situation would be made worse during the test, and, would the braking improve overall, after the test.  My thinking here was that the check valve action inside the Master Cylinder, might be hung up from storage, and maybe some repeated hard application of the brakes, would free it up.

Took the car out and performed said test.  The car braked perfectly, and repeatedly (did about 8-10 tests, braking from 50-60mph, down to 30ish, as hard as possible, without actually skidding the front tires.  No issues at all.  Since that test, the brake dragging has dramatically improved, where originally; driving around town, the rears would be dragging heavily after 3 or 4 stop signs, now the dragging is barely perceivable.  Still not 100% perfect, but definitely a huge difference. 

So I'll swap in the 4 new hoses, do a flush and bleed, and see how it functions (and looks on the temp gun at the calipers).  If it remains the same, I'll replace the Master Cylinder.

I also ordered a new set of tires because sadly, my beautiful Goodyear Invicta GL "Double Whites", while brand new, are destroyed from 10yrs of storage.  Would have loved to do the Uniroyals that are available, but after reading about how the whitewalls go brown and won't clean up, I opted for a different brand.  Wasn't sure I could fit 205's, even though I want the largest diameter I can find, for gear ratio/freeway driving happiness, so I stuck with 195/75R-14.
Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #6 on: July 28, 2022, 19:46:28 »
Brake pads get stuck in their calipers and can cause all sorts of problems. Remove the brake pads, clean everything, and then install in the same spots as removed.
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Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #7 on: July 29, 2022, 00:03:27 »
Brake pads get stuck in their calipers and can cause all sorts of problems. Remove the brake pads, clean everything, and then install in the same spots as removed.
Absolutely true.  I rebuilt the left rear caliper, replaced the incorrect right rear caliper with a rebuilt one, and rebuilt the right front caliper in Mar'20.  I replaced the rear pads and rotors at that time.  I think I re-used the front pads, because they were low mileage, having been replaced in the years prior to storage in '09.  The car doesn't get lot of mileage, so they were still new looking, and metallic pads from a top source (I'll re-inspect pad movement/backing plate condition when I install the new hoses). 

Once the new hoses are installed, I'll drive it around town through a bunch of repeated stops, and then shoot the caliper and rotor temps, and see if any are significantly hotter than the others, or if both rears are still getting hotter than than the fronts, etc.  Apologies for not having more intel due to the hoses not arriving yet.

Quick update:
The tires, brake hoses, and a new gas cap gasket arrived.  Front wheels are currently at the tire store getting re-shod.  I removed and blew through the front brake hoses.  They were perfectly fine.  I replaced them anyway.  As soon as the front wheels are done, I'll take the rears to be re-shod, and replace the rear hoses.
« Last Edit: July 29, 2022, 21:15:19 by austinado16 »
Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #8 on: July 30, 2022, 02:02:56 »

The brakes are fixed.

None of the hoses were bad, and the one that autohausaz specs for the left rear, (male end that goes into the proportioning valve, female end that goes to the brake pipe on the left swing axle) is too short at 9.75".   The hose I'd previously installed was 13.5" and is shown in the listings for later W111/W108 models.

None of the calipers or pads were hanging up.

I was able to reproduce the dragging rear brakes, with the wheels off.  Each caliper would release if I cracked just one bleed screw.  Suspecting either a failed proportioning valve, or a check valve in the master, I disconnected the rear brake line, at the proportioning valve, and at the master.  I used my MityVac and sucked through the line, from the master end, in order to dislodge anything that might be in it.  Once I confirmed that it was clear, I focused my attention on the master.  Mine has the large hex bolt fitting, where the rear brake line attaches.  I figured this must be some sort of check valve.  Using a 22mm socket, I removed the hex fitting.  Inside the master, I found a check valve assembly and removed it.  I carefully cleaned it, and was able to "sort of" confirm that it functioned.  I stopped at this point, re-assembled the car, and bled the brakes.  Immediately there was a difference in how the pedal felt, AND I could hear the rear calipers making a slight "clank" as they grabbed and let go of, the rotors.   Prior to this, with the rear wheels removed, I could step on the brake pedal repeatedly, it would get really firm at the bottom of it's "brakes are fully applied" travel, and when I'd get out to test spin just the rotors (no tires on), the rotors were completely locked.  After re-assembly, the brake pedal didn't have a feeling of getting firmer and firmer, and the rotors could easily be rotated by hand.

I gave the car a test run around town, stop light to stop light, stop sign to stop sign.  Normally after a couple of these in a row, I would feel the back of the car dragging, and the car would not creep if I took my foot off the brake, at a stop sign, or light.  During THIS test drive, there were no issues with the brakes at all, the car coasted beautifully, and bonus....  it accelerated so much better because it's wasn't using 20hp just to overcome the stuck and overheated brakes.

So with new white wall 195/75R-14's, the roof and trunk repainted, the brake system fixed, the right fog light redone inside(so it doesn't look rusty through the glass), and some major delamination of the right upper window wood bow repaired, the car is back to being alive.  I can start enjoying it, and doing more cosmetic work.
« Last Edit: July 30, 2022, 23:56:10 by Todd »
Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #9 on: July 30, 2022, 12:54:19 »
Well done - beautiful car!  Lee
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Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #10 on: July 30, 2022, 13:12:51 »
Thanks Lee.  Feels good to have it back to normal driving status.

It photographs well, but it's in terrible condition, inside and out, with horrible structural rot.
Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #11 on: July 30, 2022, 21:55:24 »
Beautiful indded.
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Todd

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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #12 on: August 01, 2022, 17:06:55 »
Thanks Wallace.
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Re: W11x rear disc brakes dragging on '66 250SE Coupe
« Reply #13 on: August 16, 2022, 21:51:35 »
Wanted to give another update:

After a couple weeks of driving it, I've been noticing that the rear brakes were starting to drag again.  Not as bad as before, and they'd release and go back to operating normally again, much sooner than before, but still noticeable, and not safe/correct.

I replaced the master cylinder and cleaned the inside of the brake fluid reservoir today, and the problem is cured.
o   Master Cylinder: Ate 03.2123-0614.3
o   Grommet Kit:     Mercedes 0005860143
Todd