Master Cylinder Experiment

[72HCODE]

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:

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

Question:

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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:

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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:

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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.

 

[72HCODE]

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.

 

[OMS]

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

 

[72HCODE]

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.

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

 

[midlife]

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

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

 

[marks73]

If you curb your drinking a little your hands won't shake and spill the DOT 3 and get on the paint.

 

[Jeff73Mach1]

But don't ease up too much or you'll get the shakes-it's a delicate balance

 

[72HCODE]

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.

 

[72HCODE]

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.

 

[72HCODE]

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.

 

[judge]

I've had two my classic cars leak with Dot 5. My 65 with stock disc brakes leaked on the July 4th weekend show four years ago. It was the first hot day I had it out. Had to put Dot 3 in it to get home. Resealed the six month old caliper suspecting bad seals. It leaked again when installed. After some bleeding. I suspect the Dot 3 had worked its way down. It hasn't leaked since.... Now my 71 is leaking Dot 5 on rear tire and floor. All those parts were new last spring. I'm thinking about soaking them in Dot 3.

 

[72HCODE]

Second day driving around.

Zero leaks.

Ok, well there might be something to this. This is the first time in 6-7 years I have not had brake fluid leaking.

So if you have a nagging leak on some part and you are using dot 5, then it is worth a shot, take the leaking part off and soak it in dot 3 for a couple of days you need to get the fluid to absorb into the rubber seals, clean out the dot 3, reinstall, bleed dot 5 and see what happens.

Worth a shot.

 

[midlife]

When I redid the complete brake system on my 1966 by replacing everything that carried or touched brake fluid (one exception: the proportion valve), I simply added DOT5, bled the MC, and tightened up a few joints. Since then, not a single leak anywhere in 7 years.

 

[72HCODE]

I wish many of us were so lucky. It isn't unheard of for this to be the case. For me I had no issues with the calipers or wheel cylinders, but the master just never stopped leaking from the rear seal. So far this trick is working for me, I hope it lasts.

When I redid the complete brake system on my 1966 by replacing everything that carried or touched brake fluid (one exception: the proportion valve), I simply added DOT5, bled the MC, and tightened up a few joints. Since then, not a single leak anywhere in 7 years.

 

[mdan575]

Talk about an interesting experiment. I like but it also kind of tells me the level of engineering while still limited that went into vehicles of that time. I didn't know that Dot 3 fluid would cause the rubber to expand enough to cause them to seal better. At the same time that leads me to think you could possibly compensate for the lack of expansion with a slightly thicker seal.

Really nice to know though since I try to use synthetic fluids over dino types when ever possible.

Cool that the seals selected were just the right size so when they were exposed to the chosen fluid of the era they sealed but they didn't buckle causing a leak. Which I have seen before. I actually had it happen to a set of big brakes. The seal inbetween the 2 halves on both calipers had expanded enough to cause it buckle inward. Needless to say I cussed out a big name company who had supposedly rebuilt them and pressure tested them for me.

 

[72HCODE]

Another follow up after a couple 100 miles now.

Zero leaks, and the pedal is nice and stiff.

 

[73vertproject]

Great thread, Dan! Thanks for the info.

 

[72HCODE]

Updating:

So it has been about a year since i tried this experiment and i want to report the long term effect:

the entire thing has been a failure.

it seemed at first the leaking did stop as i did not have a flood of brake fluid running down the back of the master.

however over time the seals must of shrunk again as i can see wetness and a drip of fluid again.

I was under the car last night and noticed the fluid collecting underneath the master where i could not see it from the top.

I'm in the middle of removing my transmission due to failure and i have to remove my drum backing plates for repair, which means i will need to rebleed my brake system again.

thus the definition of insanity is doing the same thing over and over and expecting a different result. as such i'm giving up on DOT 5 brake fluid completely.

I will be flushing my system with dot 3 or 4 a couple of times to get the dot 5 out and go from there and see if the leaks finally stop.

so i wanted to update all my threads with this new information that soaking the seals in dot 3 or 4 and installing it back on the car to stop leaking by expanding the rubber seals is a temporary fix and apparently does not work long term.

Well i tried.

this makes two things i was very positive for at the beginning and long term they cause more problems then they are worth.

1) Stainless steel brake lines, STAY AWAY FROM THEM

2) Dot 5 brake fluid in a classic car, it just leaks and leaks no matter what you try.

 

[c9zx]

Thanks for the update. It will help someone. For what it is worth, I agree with your conclusions. Chuck