I rebuilt my front calipers approximately 18 months ago. Thought I had cleaned and polished pistons, bores and seal grooves sufficiently before reassembly.

Last week the car started pulling under braking and have now found a leaky seal at one of the small pistons.

When I removed the caliper I found hat corrosion in that piston seal groove apparently left impressions (similar to pin holes) in the upper and outer seal surfaces.

I cleaned up the groove with circular Dremel hard and soft disks. I guess dreaming that would work with the groove clean I reinstalled that seal and reassembled but it didn't seal.

Does anyone know the dimension of the seals so that I may search for a single seal rather than purchase a complete kit again. I suppose I could find a dial caliper and measure but thought maybe someone had that information available and the size was available as common seals through hydraulic parts.

I know it's a good possibility and best to again rebuild the fronts and purchase new kits but wanted to avoid that until necessary.

Thanks in advance for any suggestions and help,

Original Post
I would tend to think that if you've removed enough material to get to the bottom of the pitting, you've removed a measurable amount of caliper sealing surface, so stock seal dimensions are not likely to ever seal properly.
Maybe time to consider finding another caliper?
Good point. Actually the seal looked worse than the groove. I probably should have used a better word than pitting for the description. The seal looked as if it had been pressed into coarse grit on the outer surfaces more representative of contamination or bad molding.

Basically what I did to clean it was much milder than a typical honing process so I believe there's little if any change in the original dimension.

New calipers would be nice. Do you know if any Mustang or other calipers are interchangeable?

I'll investigate replacements but am considering selling the car want to leave those upgrades to the next owner.
Just for my curiousity...
could one measure the seal dimensions?
cross section, ID, OD
I would ASSUME they are square cut orings
Thanks Larry.

Richard, yes correct square and I believe easier to measure piston and groove and more accurate, other than the o ring thickness.
Bob, I found that almost ALL Euro cars from the '70s and '80s (Cobra, Ferrari, Pantera, Porsche etc) use nickel plated mild steel pistons instead of stainless like in U.S cars since the '80s. The pistons are magnetic. So the plating sometimes pits like a rusty bumper and will tear up new seals no matter how carefully you install them. If that's your problem, the fix is new pistons if possible or unpitted-used if new are unavailable.

Your calipers are English 4-piston Girlings and are no longer sold by Girling after 45 years. No known cross-reference. Someone in Europe advertised new pistons F/S at one time. I made some on a lathe out of stainless bar stock for my ATS rear calipers- also were unavailable. Mustang calipers aren't in the same league and will not bolt on.
Originally posted by Bosswrench:

Your calipers are English 4-piston Girlings and are no longer sold by Girling after 45 years. No known cross-reference. Someone in Europe advertised new pistons F/S at one time. I made some on a lathe out of stainless bar stock for my ATS rear calipers- also were unavailable. Mustang calipers aren't in the same league and will not bolt on.

Boss tell y'a about your ATS. A rare car for sure

Thanks for the info. That particular piston does have a very minor pit but it is outside the seal area so should not affect it presently. New pads may change that though.

Maybe I can get my friend with a lathe to cut some new pistons eventually like you did.

Id there a picture of your ATS posted?

Actually, those 'ATS' calipers are stock aluminum two-piston units for a 1970 911S. I adapted them to the rear uprights and balanced them against stock Girling front calipers first, then Wilwood Superlite 2s, both with a manual proportioning valve.

The Wilwoods went on after we lost the stock fronts on a 1000 mile Fun Rally round trip to 'Vegas & back. My wife drove the mountain route entirely on the rear brakes and said the brake pedal was 'a bit long in travel' but she had no trouble keeping up with the other dozen Panteras in the group. We trade off driving; I drove home the same way and also had no trouble.

I've since added Porterfield R-4S pads & Speed-bleeders on both ends, after the ATS sprung a leak. I used 312 stainless bar stock to make 4 pistons after I found both the Girlings and ATS calipers had plated pistons. I checked an aluminum Girling Mangusta caliper last year (same as big-block Cobras) and its pistons were also magnetic.... Change brake fluid EVERY year!
No ss pistons that I know of. Simple to make but labor intensive. Roland in Germany said he had a few NOS pistons (unknown source). Girling sold off all their 40+ yr old patterns & tooling decades ago to private shops in England- for both early & late Pantera. Never heard anything more about spares. ATS did the same- last contact I had from them was a shop in S. Africa. VERY small market...
That repair kit with new pistons is a possible option if I keep having issues.

Initial inspection does not indicate leaking fluid on that caliper now although I have not removed it.

The car still pulls left under hard braking after an attempt to bleed the brakes again. I used the do it yourself method using a hose on the bleeder submerged into a bottle with fluid. This time I'll get someone to pressure them when I bleed.

I will pull the caliper and check it again for leaking seals or sticking pistons. I can't imagine anything else would cause it to pull left. Any fluid residue on the pads should be gone by now after cleaning and running.

Seemed initially obvious why with the leaking seal but now am wondering if something else is causing the left pull.
I had the same problem as you described, leaking caliper, started pulling to one side.

What I found was the fluid had saturated and swelled up the pad material on one of the pads, and I wasn't getting good braking power on the leaky side.

The one affected pad was swollen in the center, and you could tell that it wasn't gripping (engaging on the rotor) the same as the other three.

The leak was on my drivers side caliper, and the car would pull to the passenger side.

I was told that you can soak the pads in brake cleaner (and/or water) and get the fluid out, and then re-bed in the pads. I had a new set of Porterfield R4S pads, but I didn't try it.

Last edited by rocky
Another reason for pulling to one side is replacement of only one rotor. Rotors should be replaced in pairs from a single mfgr. Cast iron is like anything else- the composition varies and so does the coefficient of friction. Try swapping rotors side to side and see if the pulling goes with it.
Thanks for sharing that experience and information. That seems to be consistent with the chain of events with me thus thus far.

I attempted to clean the pad previously by just washing the face with gasoline but the fluid may have penetrated the pad so I'll inspect the pad and try the cleaning method you suggest after I remove the caliper and verify there is not leak.

I have not changed rotors and agree that would cause an issue. I'm willing to attempt swapping rotors around if pad cleaning doesn't correct it in an attempt to discover the cause.

Thanks guys for the suggestions

Just finished removing the caliper. No apparent seal leaking so happy that the seal I removed and reinstalled is holding. Pistons move freely.

I'm letting the pads soak now and will attempt to treat the surface when I can get back to them.

I'll update later with results.
I didn't have much fluid leaking but any is too much obviously. Haven't heard of such a heat tool.

I'll see how the soaking and cleaning goes.

I gave the seal a good pressure test with the boot off and found no seepage.

Believe I need to do a better bleeding process also.
I sent you a PM with my address. Just wondering if you received it and sent the seal(s). Apparently there is still some seepage although I thought it was holding.

The one in question is for the small piston.

Please excuse me if as a beggar I seem impatient or demanding.

Plaintiff’s Lawyer “…could you tell the court how much you saved by performing the “internet shade tree remedy” on the part you didn’t want to place the day before you veered into the children’s play ground?”

Appreciate you sending the seal. I'm going to see if I can match with one from a parts supplier.

I'll be in touch.

Thanks for your help,

You make good points regarding the salvage of parts rather than replacement for both costs and safety reasons.

My pad in particular had very little exposure to fluid and I just wanted to clean the surface properly since my pads are in good shape.

As you know sometimes it's the challenge and satisfaction you get from repairing rather than replacing regardless of costs but your points are well taken.

Larry was kind enough to send me a new seal and I want to find if there are seals available without buying new kits since I rebuilt calipers only a few months ago, and in the event another fails. I hope my issue with that one seal was my failure to properly prepare the the groove before installing the new seal since no others have failed (as of yet).

Costs for new kits aren't outrageous from some vendors when you consider we are driving vintage sports cars. A rubber o ring if available is a lot cheaper than a new set of kits though when only one or two o rings are required.

Regarding new pads, what do you recommend for stock calipers?

Thanks for the chart and comments

Maybe some one knows for sure what parts kits are made from. looks like EDPM would be a good choice

BTW...You do know the square cut is used to allow the piston to "pull back" when pedal is released.
( I wonder if the quad would give more pull back?)

(intersting, i just googled and used the best looking compatibility chart, however I noticed it shows SBR excellent for mineral oils where I thought it was poor)


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I finally got around to swapping the pads left and right since the car was still pulling to the left.

Yep! She pulls right now as I hoped it would. Amazing how little fluid it takes to ruin a pad.

New pads are on the way but I couldn't find any at a price of $8 though.

Thanks for all the advice and especially to Larry for the seal.

Originally posted by JFB #05177:
Plaintiff’s Lawyer “…could you tell the court how much you saved by performing the “internet shade tree remedy” on the part you didn’t want to place the day before you veered into the children’s play ground?”

That gave me a well needed chuckle for the day! Big Grin
You are like the Sherlock Holmes of brake mechanics.

Congrats on finding (and solving) the problem.

Looking for small caliper seals. Very hard, if not impossible to find.

12.66 O.D. x 8mm I.D. x 2.0 mm thick. Square cut.
Should be EPDM material. If these can't be had what other option? And no, replacing the whole caliper is not an option.
What I cannot find anywhere here on this board, is the stock piston diameters and the seal paint codes for the Girling calipers front or rear.

Girling pistons were generally standard sizes, not yet metric, and the seals had a three color paint code on the backsides of the seal.

Once you have this information, you can successfully bug Girling distributors for parts until they go mad....!

One last question: The front calipers must be parted in order to get the pistons out? You can't sneak them out without busting the seals on the x-over lines? (On the Mangusta with the HUGE pistons up front....there is no need to bust the caliper apart....the pistons pop out...)

FWIW on this rather old thread, Larry Stock or Pantera Parts Connection in Carson City, NV offers rebuilt stock front & rear calipers using reconditioned castings & pistons, along with new lines & other parts as needed. I think he also has OEM-type brake pads. OEM pads seem to be made of an extremely hard friction compound that lasts like iron but will not stop the car like your Porterfield R-4S pads. Being bonded linings, working too hard on removing leaked brake fluid may well also debond the glued-on friction linings from the steel backing plates. That will prove exciting in a panic stop when you really need them.

There are two different types of OEM pads, too; one has laterally drilled holes for wires used to detect badly worn pads by grounding the wire to the rotor, which turns on the red brake warning light on the dash. Our '72 L had such pads. Later OEM replacements do not have wires. If necessary for extreme show purposes, any pads can be drilled for fresh wires; originals had linen insulation typical of '50s english cars.

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