PSA do NOT put petroleum based fluids in your master cylinder reservoir

mrblaine

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Quail Valley, CA
Bottom is stock
Middle is Durango
Top is the problem child
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Pushrod seal, left is good, right is not good.
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Another view of just how much the seal expanded. It is supposed to be the size of the one beside it.

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Cap seal expansion which is the dead giveaway that you have a problem. Right is good, left is not so good.
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I would have pics of the secondary piston and seal but they are locked up hard in the bore. Air pressure just bypasses and beating it hard on the concrete won't even budge it.
 
Wait, for an idiot like me, isn’t standard brake fluid the only thing that should go in a master cylinder?

I’ve never ever heard of anything else being put in there. I know very little, and couldn’t imagine putting something else in there.
 
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Wait, for an idiot like me, isn’t standard brake fluid the only thing that should go in a master cylinder?

I’ve never ever heard of anything else being put in there. I know very little, and couldn’t imagine putting something else in there.

Clear power steering fluid is the most common culprit. But yes, Dot 3, Dot 4, and Dot 5.1 are the standards. There is a Dot 5 which is silicone based. They are all forward and backward compatible except Dot 5 which can't be mixed but can be used in a well flushed system with no ill effect.
 
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Clear power steering fluid is the most common culprit. But yes, Dot 3, Dot 4, and Dot 5.1 are the standards. There is a Dot 5 which is silicone based. They are all forward and backward compatible except Dot 5 which can't be mixed but can be used in a well flushed system with no ill effect.

Yes sir. We use dot 5 In our electric lift brakes, well the older ones anyway before every thing went to electric and regenerative braking. It’s always fun to deal with the systems after some ass hat mixes dot 3 & 5. Sometimes they play ok together in a dot 5 system. Other times they cause a lot of problems.
 
Yes sir. We use dot 5 In our electric lift brakes, well the older ones anyway before every thing went to electric and regenerative braking. It’s always fun to deal with the systems after some ass hat mixes dot 3 & 5. Sometimes they play ok together in a dot 5 system. Other times they cause a lot of problems.

Basically they are depending on zero maintenance for the brakes then.
 
Basically they are depending on zero maintenance for the brakes then.

I heard the silicone style helps reduce moisture? The brake is an automatic/spring applied parking brake/emergency brake. Most all normal use braking is done with reversing the motors. Possibly has to do with freezer trucks as well.
 
Also the dot 5 stays nice and clean/purple until there is a contaminant added to the system. Is the life of dot 5 longer?
But that is irrelevant in this conversation. Lol. DON’T USE IT IN YOUR JEEP.
 
I heard the silicone style helps reduce moisture? The brake is an automatic/spring applied parking brake/emergency brake. Most all normal use braking is done with reversing the motors. Possibly has to do with freezer trucks as well.
Silicone is not water miscible. The rest are hygroscopic by design. The water is absorbed into the fluid to slow down corrosion of the components. If water gets into the Dot 5 system, it just settles in the low places and does its thing.
 
Also the dot 5 stays nice and clean/purple until there is a contaminant added to the system. Is the life of dot 5 longer?
But that is irrelevant in this conversation. Lol. DON’T USE IT IN YOUR JEEP.

You could use Dot 5 if you flush the system since it doesn't mix with the others. You can but there is no benefit.
 
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I witnessed this one time. Started a new job, took the bucket truck out. Started to lose brakes. Pulled over and pulled the lid off the master. The rubber lid seal came ballooning out at me, I swear it was twice the size it should have been. My first guess was someone added powerstearing fluid to it…. That truck was so neglected it didn’t surprise me unfortunately.
 
Silicone is not water miscible. The rest are hygroscopic by design. The water is absorbed into the fluid to slow down corrosion of the components. If water gets into the Dot 5 system, it just settles in the low places and does its thing.

You mean hydroscopic? 🤣 Your old post about that word and engineers not knowing how it was spelled or pronounced still cracks me up sometimes.
 
By the looks of the pictures I’d say you caught that fairly early. Sometimes when you take the cap off the master cylinder the rubber seal on the bottom of the cap will be 3 times it’s normal size. Power steering fluid does wonders
 
By the looks of the pictures I’d say you caught that fairly early. Sometimes when you take the cap off the master cylinder the rubber seal on the bottom of the cap will be 3 times it’s normal size. Power steering fluid does wonders

Not mine. It was on a big brake kit install. Customer had various complaints that did not make any sense but all pointed to a bad install. Except the shop had the rig for 3 weeks and it was fine during the break in and test drives for that length of time after install. Customer picks up rig, after about a week starts complaining about how it works or doesn't work. Customer accuses shop of using Dot 3 brake fluid instead of Dot 4. Customer flushes system and puts in Dot 4 with a new master that "miraculously" solves the problem.
 
Clear power steering fluid is the most common culprit. But yes, Dot 3, Dot 4, and Dot 5.1 are the standards. There is a Dot 5 which is silicone based. They are all forward and backward compatible except Dot 5 which can't be mixed but can be used in a well flushed system with no ill effect.

I've replaced entire brake systems at the expense of some local quick lubes. It's not pretty what the wrong fluid can do, and how fast it can do it.
 
You mean hydroscopic? 🤣 Your old post about that word and engineers not knowing how it was spelled or pronounced still cracks me up sometimes.

You had me worried you didn't know what you were talking about there until I read and processed the rest of your post.
 
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I've replaced entire brake systems at the expense of some local quick lubes. It's not pretty what the wrong fluid can do, and how fast it can do it.

I explained that very carefully without trying to point fingers. My response to the brake fluid mix-up claim was the absolute dead giveaway that something other than brake fluid being introduced into the system was the swelling of the cap seal. That is without exception, period.
 
I would have pics of the secondary piston and seal but they are locked up hard in the bore. Air pressure just bypasses and beating it hard on the concrete won't even budge it.

You know what they say. If you first don't succeed, get a bigger hammer.
 
I explained that very carefully without trying to point fingers. My response to the brake fluid mix-up claim was the absolute dead giveaway that something other than brake fluid being introduced into the system was the swelling of the cap seal. That is without exception, period.

About 20 years ago, my mothers aunt and uncle were killed in an accident in California. A Chevy dealership serviced their Suburban and topped off the brake fluid with trans fluid.