[xk] Dunlop brake caliper question

My front end suspension rebuild is complete except for a hitch with the brakes. I decided to clean up the Dunlop calipers after being unable to get any fluid out of the bleeder. Turns out the little ball under the bleeder screw was stuck tight in the hole. Everything is clean now but during reassembly, I had a problem. The piston doesn’t slip into the bore, and over the “retractor pin”. I measured the diameter to see if it is square or rectangular in shape. It isn’t; .128” diameter. So, the hole in the back of the brake piston must be round as well. After reviewing the archives, it seems this little pin seemed like a good idea at the time, but may have caused more problems than it solved. Lots of speculation on what the purpose is; piston alignment, is one theory. Pulling the piston back after pressure is released is another, hence the “retractor” term in the name.

I need some advice on how to get the piston back into the bore, and over the pin. There is a slight bevel at the outer edge of the bore that makes it fairly simple to get the rubber seal started. The problem comes up when the pin first meets the hole in the piston. Even with the slightest pressure, it gets stuck and is difficult to pull off again. Sort of like the Chinese finger traps we played with as kids. Clearly, the hole and the pin go together, but I’m missing something in the process.

Anyone been at this stage and figured out "the secret handshake”? Thanks for any help.

Aloha, Rob

Aloha Rob,

That’s how it is supposed to behave. From the manual:

“Locate the piston assembly on the end of the retractor pin. With the aid
of a hand press apply an even pressure to the backing plate and press the
assembly into the cylinder bore.”

It’s not supposed to be easy. It takes hydraulic pressure to push the
piston out against the drag of the retractor bush, which grips the pin
inside the piston.

Mike Eck
New Jersey, USA

'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

My front end suspension rebuild is complete except for a hitch with the
brakes. I
decided to clean up the Dunlop calipers after being unable to get any
fluid out of
the bleeder. Turns out the little ball under the bleeder screw was stuck
tight in the
hole. Everything is clean now but during reassembly, I had a problem. The
piston
doesn’t slip into the bore, and over the “retractor pin”. I measured the
diameter to
see if it is square or rectangular in shape. It isn’t; .128" diameter. So,
the hole in the
back of the brake piston must be round as well. After reviewing the
archives, it
seems this little pin seemed like a good idea at the time, but may have
caused
more problems than it solved. Lots of speculation on what the purpose is;
piston
alignment, is one theory. Pulling the piston back after pressure is
released is
another, hence the “retractor” term in the name.

I need some advice on how to get the piston back into the bore, and over
the pin.
There is a slight bevel at the outer edge of the bore that makes it fairly
simple to get
the rubber seal started. The problem comes up when the pin first meets the
hole in
the piston. Even with the slightest pressure, it gets stuck and is
difficult to pull off
again. Sort of like the Chinese finger traps we played with as kids.
Clearly, the hole
and the pin go together, but I’m missing something in the process.

Anyone been at this stage and figured out “the secret handshake”? Thanks
for any> help.

Aloha, Rob

In reply to a message from rob caveney sent Mon 11 May 2015:

in answer to ‘‘My front end suspension rebuild is complete
except for a hitch with the brakes’’. I too am working on
Dunlop calipers and plan on rebuilding mine if possible. I
found some great info on the web about these calipers and
disk brakes in general. Try
http://www.acarplace.com/cars/brakes.html also
http://mgaguru.com/mgtech/twincam/tc207.htm The pin you
are talking about along with spring washers at it’s base
and the ‘‘retractor bushing’’ (the Chinese Finger Trap) act
together to bring the piston back after release of
pressure. You just need to use pressure to overcome the
retention of the retractor bushing to install the piston.
My problem is that I have some pitting in the caliper walls
and would love to know if anyone has some good rear Dunlop
calipers they would part with or if anyone knows an
inexpensive place to rebuild or buy new. If you have such
info please email me at dan.kaitschuck@gmail.com.

thanks,Dan K-chuck–
Dan K
San Tan Valley, AZ, United States
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In reply to a message from Dan K-chuck sent Thu 11 Jun 2015:

Dan you should consider the modern replacements. I had my
original Dunlop callipers sleeved in SS and whilst they
are ok the new items are simply superior. Look up BG
developments on Google–
Phil.D 3.8 etype, XJR6, XK150 FHC, 2.2 diesel Xtype
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Well, Dan, I suppose it depends on what you mean by “inexpensive”.

About 15 or so years ago I was redoing a friend’s SI 4.2 XKE calipers. I sent his cylinders to, IIRC, White Post and had them sleeved in brass for $25 a hole (more to the story, but …). They were fine, but, IMHO, were too expensive coming out to $100 a set for fronts or rears. Kits were pretty expensive as this was before ebay got into full swing and they were only available from the usuals. That came out to about $150 or $200 a front or rear set not including labor. The last time I checked, prices were up to $60 a hole everywhere and you can guess what my sentiments are now. Inexpensive is a local definitions existing only within one’s mind.

Recently, I took apart some of Girling or Dunlop units I got from ebay which I bought (for my version of) extremely cheaply for parts only. They were crudded up (technical term) and I had to use the grease gun ploy to extract the pistons. The prospect was looking even more dim than I had imagined and I thought I had been royally “rogered” even at cheapskate prices. After seeing off the great globs of grease, curiously, I began to clean them further. There were six of them and five of them had near perfect original bores and only required some very light dressing up!

Yeah, I don’t understand it either, they’re usually total write offs only good for sleeving.

Another set I bought for the same purpose, when cleaned, had already been sleeved in brass to my utter amazement. You never know.

So, no there are no inexpensive places to sleeve or buy new anything. They won’t even give you a volume discount. Sorry.

BTW, I just put the old “perfect bore set” on my work-in-progress '53 120 FHC. The sleeved set I passed on to a friend.

Best regards, Brian

"… or if anyone knows an inexpensive place to rebuild or buy new.