I attended the Carlisle Import Car Show last weekend and found 2 brake proportioning valves in a junk pile.
i cleaned them up, took them apart and inspected the pistons and seals.  They both looked OK and
better than mine.  One had the shorter spring.

so... for now I am OK.

when running and the rear brakes lock up I will revisit replacing the brake proportioning valve.

thanks all for your feedback.

Thanks for the feedback.

Problem finding supplier for EPDM square cut o-rings - AS568A-202 and AS568A-205.
either not available in EPDM, min qty too high, or min order $ too high.

this may need to wait till the car is actually running; only then will I need to stop!

i will try to find a good image of the shaft and upload for all to see along with my own shaft and seals.

I wish to offer additional cautionary notes to those from Chuck.

Don't be tempted to use o-rings from the local parts store.  The guys there are likely to have very little familiarity with what materials are compatible.  Instead look to industrial suppliers who can provide you seals made of EPDM.  EPDM will be compatible with all the brake fluids in common use.  Common o-rings are typically made of Buna-N (Nitrile) which will swell in DOT-3 and DOT-4.

As for size, Imperial sized o-rings follow the AS568A standard.  They actually measure a bit off from their nominal values.  You mentioned 1/4" ID with a 1/8" section.  That would be size AS568A-202 (actual ID = 0.234, actual OD = 0.512).  The "-202" number in EPDM is what you need to ask for.

Also, be cautious using a NOS brake kit containing 50 years old rubber parts.  I would never consider using such old components without carefully inspecting them.  Lastly, even if the kit's seals look good, if they were made before the mid-1970s, the seals will still be made from natural rubber which is not compatible with DOT-3.  Use DOT-4 only if you use the rebuild kit seals.

The complete valve assembly 21A1774 on Host site is pictured as black.
i believe the original color is gold (brass like).
can someone confirm?

I have the original spring and copper washer which look good.

I was able to remove one of the washers/seals.
this is similar to a master cylinder washer/seal.
O/D = 1/2”
I/D = 1/4”
thickness = 1/8 “


the second washer/seal is more interesting!
if sits inside a metal ring which is removable.
it is rather gummy and difficult to remove!
the approx dimensions are
O/D= 3/4 ”
I/D = 10/16 “
thickness = 1/8 - 3/16 “

i will check a hardware / auto store and see what I can find.

Be careful looking for kits. As I recall, the spring was different (stronger?) on the S than Minis without the 7.5" discs.

Thanks for the feedback.

i had checked eBay and did see the rebuild kit SSB802.

i was hoping to replace the seals for a few dollars (similar to master cylinders) rather than purchase the rebuild kit for $36.

 I had also checked with various suppliers as well.

do you happen to know the seal/washer sizes?
if not I may try to take them off and go to an auto parts or hardware store.
if I strike out I can buy the rebuild kit.

the car is nowhere near being drivable so I will not be able to test my existing valve.

https://www.ebay.co.uk/sch/Vehicle-Parts-Accessories/131090/i.html?_from=R40&_nkw=ssb+802

roger.  did you look at www.ebay.co.uk in England?? ??later bc

The repair kit includes the entire plunger. I don't think seals are available separately. FYI the Lockheed part number for the plunger is SSB 802. Symptoms of failure of the unit are the rears locking up under hard braking.

66S

just took apart my brake proportioning valve and it looks like I could use new seals on the plunger.
i do not see that these seals are available from suppliers.
Can anyone help re: specs for these seals? 
I do not want to remove/destroy until I absolutely have to.
would like not to purchase new valve.