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Master Cylinder Experiment
#1
Here is one for the books.

I have no idea how this will end.

The problem:

I used Dot 5 brake fluid when i rebuilt my brake system. From day one my master cylinder rear seal leaked down the front of my vacuum booster.

There were other issues with the system that were resolved over the years but the nagging master leak still continued.

I replaced the master 5 times, with 2 different manufactures and used Re-manufactured and bran-new masters, no matter all of them leaked from the rear seal. The Re-mans tended to leak less then the new ones.

We know the system and the seals in our cars are rated for DOT 3/4/5.1 fluid which are glycol based. Dot 5 is silicon based.

Switching back and forth from 3/4/5.1 and dot 5 is not easy and due to incompatibility it may require changing out all rubber components of a brake system. flushing with Alcohol is an option but there can still be problems.

I have never been a fan of DOT 3/4 since it eats paint.

Problem:
--------
You have seals rated for dot 3/4 and things leak with dot 5, but you really want to use DOT 5.

Question:
---------
What would happen if you removed the rear piston from the master where the leak occurs, soak the seals in DOT3/4, clean the piston, and reinstall? Would the Seals Swell in reaction to DOT3/4 and maintain the swell even after DOT 3/4 was removed from the system?

Observation:
------------
When i went through my Distribution valve, which had used DOT 3 for 40 years and switched to DOT 5 i never seemed to have an issue with it leaking internally. The Distribution valve requires a seal between the front and rear brake systems and has a balance sensor which tells you if you have a leak in the system caused by a pressure imbalance.
During prop valve rebuild i ran across another unit which had internal problems. When i opened up the unit with issues i discovered the rubber had Swollen to 2 times its original size compared to the first unit. The swelling was caused by unknown factors, but the defective unit used DOT 3 fluid.


Hypothesis:
-----------
Dot 3 will soak into the rubber and expand them slightly and they will remain slightly expanded and thus stop leaking even in a DOT 5 environment. The idea is by expanding just the 2 seals on the master that leak, i will be able to avoid a brake flush and possible change of working components being forced to use only DOT 3 or 4 or 5.1 fluid.

Experiment:
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Take a known Leaking DOT 3 rated spare Master Cylinder that was bench bled with DOT 5 from day one. remove Rear piston, Clean and remove DOT 5 using 100% isopropyl alcohol . submerge cleaned piston with rubber seals in quality DOT 3 brake fluid for a couple of days, Clean Piston again, reinstall into same master, Bench bleed and Observe rear seal for leaks over time.


*Step one completed, located old Master that is known to leak with DOT 5

*Step two completed, Removed rear piston with seals, inspected bore for any damage. Bore is Smooth. inspect piston Seals, Piston seals appear bran-new.

*Step Three completed, clean piston completely of DOT 5 fluid.

next steps to complete.
Buy dot 3 fluid, Soak piston in dot 3 for a couple of days, re-clean, reinstall and observe.

final step would be installing on Vehicle and observing outcome in garage setting.

Right now i have another DOT 5 master on the vehicle and it is weep leaking from the rear seal.


Note this is all an Experiment i am curious about the outcome.

stay tuned.
  Reply
#2
Prestone synthetic dot 3 bought.

Used calipers to measure rubber outer seal lip before dip.
25mm

Piston and seals went into dot 3 bath at 12pm June 30th.
  Reply
#3
72HCODE;186051 Wrote:Prestone synthetic dot 3 bought.

Used calipers to measure rubber outer seal lip before dip.
25mm

Piston and seals went into dot 3 bath at 12pm June 30th.

Dan, there is your problem...
Nothing on a 71-3 Mustang is in MM Tongue
  Reply
#4
But my car is mostly made in china now, lol

Took my car for a spin put about 30 miles on it, the vacuum booster has a lovely wet pool coming from the rear seal.



OMS;186060 Wrote:
72HCODE;186051 Wrote:Prestone synthetic dot 3 bought.

Used calipers to measure rubber outer seal lip before dip.
25mm

Piston and seals went into dot 3 bath at 12pm June 30th.

Dan, there is your problem...
Nothing on a 71-3 Mustang is in MM Tongue
  Reply
#5
OMS;186060 Wrote:
72HCODE;186051 Wrote:Prestone synthetic dot 3 bought.

Used calipers to measure rubber outer seal lip before dip.
25mm

Piston and seals went into dot 3 bath at 12pm June 30th.

Dan, there is your problem...
Nothing on a 71-3 Mustang is in MM Tongue

Yup, Mustang Monthly ignores the 71-73's.

Let me check your shorts!
http://midlifeharness.com

[Image: Flamicon2.jpg]


[Image: oldfart.png]
  Reply
#6
If you curb your drinking a little your hands won't shake and spill the DOT 3 and get on the paint. Tongue

[Image: 2rr7aiv.png]

Just cruising along minding our own business when BAM!!! The LAWS show up.
  Reply
#7
But don't ease up too much or you'll get the shakes-it's a delicate balance

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
  Reply
#8
Piston removed from dot 3 at 8:30pm today.

Inspection looked the same, caliper showed very slight change.

Cleaned 100% alcohol.

Reinstalled piston in master, noticeable difference when reinstalled felt tighter.

Dot 5 refill.

Bench bleed went way different then before, can't explain it but noticeable difference when air was pushed out.

Cleaned outside surface of all overfill to inspect for leaks, pushed piston a few times did not see a weep leak with flashlight. I will cycle the piston over the next few days making sure not to clean any leaking dot 5 so I can see what is leaking.

Final test will be installing on car and seeing effects.
  Reply
#9
Master on the test bench was not weep leaking from the rear seal, kept it for about 2 days on the bench.

Decided to install on car last night.

Test master installed and system bled with dot 5.
Held brake for a couple of minutes under full pressure and did not see rear seal leak like before.


Test drive will be soon, rain today.

Let's see if it leaks or not in a full scale test.
  Reply
#10
put over 100 miles on the car today, in the 80s had the A/C compressor going.

came home, and had my hesitation when i opened the hood, shined a light under the master on the booster.

IT WAS 100% DRY!!!!!!!

amazing! i had no leaks around the master other then one line that i needed to tighten a little more.

i will continue testing.

but this is the first time my booster has been dry under the weep hole running DOT 5 fluid.
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