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Master Cylinder leaking from the top

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Hey guys and gals, I sitting here scratching my head.


I have a master cylinder on a 91 Comanche that just keeps leaking out the top like a son of a gun.  I’ve spent about the last 3 days trying to figure it out.  I’ve read all the write ups from this AWESOME website but still no luck.  The weird thing is only the front reservoir for the rear brakes keeps running low.  Break fluid had already stripped a good amount of paint...and I’ve gone through 2 bottles of Dot 3.


1.  I’ve doubled, tripled, and quadrupled checked all the MC fittings....no leaks

2.  Connection from MC to booster....no leak

3.  Booster no leak

3.  Proportioning valve no leak

4.  Rear drum breaks no leak

5. Front calipers no leak

6.  Height sensing valve no leak


break lines check out, I’ve bleed the break system twice just for good measure and everything is checking out.


I am positive it’s leaking from the top.


I cleaned the lid edge of the MC and took all the crap off, it’s down to very clean metal.  I’ve even checked to see if the MC as well as the lid is just warped but it’s all good.  I also tightened down the wire that woods the lid down.


Is it as simple as the seal?  It’s a brand new seal, it’s not hard or cracked.  I just don’t freakin know but I don’t want anymore stripped paint and no more morning Dot 3 top offs.


any ideas?  

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Put on some safety glasses, open up the reservoir, and get a helper to step on the brakes. If fluid squirts out of the front compartment into the rear, replace the master cylinder. 


As as to the leak, find a dry patch of ground (put down cardboard, or something) and absolutely stand on the brakes a few times. Both feet if you need to. The fluid doesn't just dissapear, it's got to be going somewhere. Or, if there's paint stripped off somewhere, it's a pretty big clue. Fluids leak downwards and backwards, so look forward and up until you stop seeing bubbling paint. 

If you've got a manual trans, the brake fluid could be coming out of the clutch master cylinder, and the rear brake reservoir could just be emptying itself because of a leak in the rear circuit somewhere. 

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I ran out of day light but I’m going to try and see if the front reservoir is squirting into the back one tomorrow.  Thanks for that tip.  Only question I have is why would that indicate that I need a new MC?  Does that mean something’s buggered internally inside the MC?


thanks again but I am pretty gosh darn close to 100% positive it’s leaking from the top of the MC cap.  I’ve spent the 

last 3 days combing over the brake system.  Tried the cardboard trick, followed the bubbled paint, etc etc etc on etc.  I scrubbed my entire engine bay clean, crawled under the truck cleaned everything.  It was BONE dry From break fluid, checked it the next morning still bone dry.  Drove it to work and checked it again and I have leakage from around the MC cap.  I also did the 2 foot test as hard as possible with my buddy and it just seems to leak from the cap while I’m driving.


i was thinking it starts leaking while under constant pressure while slowing down at stop signs etc? 


Also I’m hesitant to replace the Mc because it’s fairly new.  

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I picked up a NOS Mopar booster, a used 95 Master cylinder, a master cylinder rebuild kit, and pre-flared length of brake line. You can even use your original flare nuts on the new line you have to bend. It's a pretty simple job - hardest part is re-bleeding the system.


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Note that you will need to relocate your windshield washer fluid reservoir to make room for the double diaphragm booster upgrade.  Hornbrod used a late model reservoir installed in the fender well (you can see the fill cap in his photo).  I got the reservoir from the same ‘96 XJ I got the double diaphragm booster from and located it on the passenger side firewall.  One advantage of the XJ reservoir is that it has two pumps.  I brought the wiring and tubing from the second pump up near the connections for the pump I’m using.  If the first pump  fails I’ll just switch the connections and keep on truckin’.

BTW - after I had installed the used booster I found it was defective and bought a rebuilt unit.  Considering all the extra work, it would have been a better move to go with new or rebuilt parts at the start of the project.

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Just replaced my Booster/MC on my 91.







Getting to this part, in order to remove was

probably the toughest part of the whole job.







When you remove the old one, the Pedal Bracket below,

might shift a bit, on you and the firewall holes will not line up.

It's a little bit of a pain to get the new one to go right in, if you

don't have a helper.

I was able to get a couple of the booster bolts to go in enough

to start a nut on the threads, then worked the rest of the bolts

through the firewall.









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Went to the junkyard today and pulled a MC and booster from a grand Cherokee, removed as a whole unit.  Pretty simple swap but have two problems.


I think the booster from the yard is bad...  I get a “squish” sound when I apply the brakes.  It feels like my braking power is the same with the stock MC/booster, I figured braking would be better after the swap.  


I need to remove the spacer that came with the grand Cherokee booster because the brake pedal isn’t pressing down on the brake light button.  I had to tape a folded up piece of cardboard on my brake pedal for the time being so my break lights work properly.  


Also while I was under the truck I noticed someone did a height sensing valve by pass, do I still need to bleed the brake lines as the FSM link suggests?  Does the New MC have to be benched bleed?


last question and thanks for all the replies, wheres a good spot to relocate the windshield wiper reservoir?


i can post photos if you want em




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A new MC will require a bench bleed to get things started.


Now that your vehicle has the height sensing valve bypassed, just bleed the brakes like a "normal" car.


Pro-Tip:   On vehicles as old as ours, pulling a power booster from a boneyard is an iffy proposition.  The rubber will be dried out and possibly cracked, and the booster may not work as intended.   As financially painful as it is, a reman'd booster is a much safer bet.

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Ok guys everything’s finally worked out.  I removed the spacer and my brake light button is working fine and re-bleed my lines and I have significant better brakes than my stock set up.  For anyone thinking about this swap I would do it.  


the things that hung me up which can be easily avoided 


1.  Make sure your brake light button is seated all the way in its bracket in front of the brake pedal.  Took me a long time to realize everything was spaced correctly but couldn’t figure out why my brake pedal wasn’t engaging the break light button.  The button was simple not seated all the way in its bracket


2.  Take your time with the brake lines from the proportioning valve to the MC if you need to bend or reroute them.


3.  Reconnect your brake booster vacuum hose lol


really appreciate the help 

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