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Leak from rear of new master cylnder

Over the weekend I installed a new master cylinder and rear wheel cylinders on my 87 Dakota, trying to remedy a slowly sinking pedal while applying light to moderate pressure, like at a stop light. There was no loss of fluid, so I figured it was the MC. It was time to replace the shoes, so I did it all at once. Bled the lines and got ready for ‘problem solved’, but that didn’t happen. After driving around today doing what I do, the problem was still there. New MC so I began to suspect a leak. Sat in the driveway with the engine running and applied enough pressure to the brake pedal to get it to sink, removed foot and repeated about 10 to 15 times, turned off engine and got out and looked for leaks on the concrete. There were drips under th MC area. Lifted the hood and found fluid dripping down the booster housing. Damn! Bad new master cylinder, I thought (not a rebuild). The leak was apparently coming from the rear of the new MC.

Then I came across this:


Master cylinder problems include external leaks, internal leaks and valve problems. Brake fluid leaking from the rear of the master cylinder bore is usually bad news because it means the seals are leaking. But sometimes a leak at the vacuum booster input seal and/or pushrod misalignment can cause a fluid leak, so a closer inspection is needed to determine the cause.
Brake Master Cylinder

This seems to indicate that the brake vacuum booster could be causing the leak, but I don’t see how and that is why I am posting this. Is anyone familiar with this? Can a bad brake booster cause a good MC cylinder to leak? If so, how? I don’t want to throw a new MC at the truck if that is not the problem.

PS - The MC and wheel cylinders that I replaced were original.


The rear of the MC where the rod goes in is sealed with a rubber O-ring. Since the brake fluid force is against the front of the pistons, a leaky rear seal is not common, but there’s a lot of force on the pushrod and if it’s not coaxial to the pistons’ rod enough lateral force could be applied to the MC to cause leakage past the rear piston seal and out the rear MC seal. The center rod could be distorted.

Recheck your work, but the MC should be returnable/replaceable under warranty if you’re having a problem, and that’s the direction I’d head. The MC may be defective, especially if it’s a low-price aftermarket MC.

Thanks so much, t_s_m. As far as my work goes, there is not much to check. The MC is bolted to two threaded studs on the booster… idiot proof, I would think. I did notice, however, when I swapped the MC, that the pushrod from the booster was quite ‘wiggly’, it was not rigid. This meant nothing to me, as this is the first MC I have changed. Could this be the cause of the fluid leak?

Yup. Great observation, by the way. Absolutely relevant.

All the MC’s I’ve changed out have had a wiggly pushrod. I don’t think that’s the problem. I expect you just got a bad MC out of the box. They aren’t supposed to leak there, but if the rear seal leaks, they will . It’s not an uncommon thing for MC’s with a lot of miles on them to leak there. But new ones can too, if they were put together in haste or a defective seal. Suggest to test the power booster to make sure it doesn’t have a vacuum leak first. If not, then if I had this problem I’d just replace the MC again.

Note: Be sure not to manipulate the MC pushrod until it is full of brake fluid, and bench bleed it before installing. Doing that can damage the seals. That’s all usually included in the instructions that come with a new MC. BTW, many “new” MC’s are actually rebuilt old ones. Even if they look new.

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I am inclined to go for a rebuilt brake booster:

Brake boosters don’t contain brake fluid, so unless you have reason to believe the booster is bad for some other reason, it won’t solve your problem.

Normal, they are all that way. If the new master cylinder is leaking from the rear seal it is defective.


I have a full-time vacuum meter hooked up inside the cab. It reads the same at idle with the booster connected to and disconnected from vacuum; stepping on the brake and not stepping on the brake, always a steady 18 inches. This tells me the booster is not leaking vacuum. And even if it were, how would this cause fluid to leak from the back of the MC?

It seems that you and t_s_m differ on the effect a wiggly booster pushrod has on the proper operation of the MC. t_s_m says it is relevant, and you say ‘not so much’.

I did, indeed, bench bleed the MC before installation.

t_s_m… still not sure why a wiggly booster pushrod could cause a new MC to leak from the rear.

If it’s too wiggly it can allow too much lateral load on the rear MC seal.
However, our difference in our opinions of how much is too much is irrelevant. The rear MC seal is leaking, and it shouldn’t be. The root cause could be as simple as a slight bend in the MC piston rod or too much lateral runout in the machining process.

One thing I wondered but didn’t state was the source/manufacturer of the MC. There are a lot of good aftermarket parts out there, many from OEM manufacturers to car makers, but there’s some junk out there too. George’s warning about fraudulent “new” assemblies is prudent too.

OK, the consensus seems to be that a wiggly booster pushrod is normal, and that a leaking MC is a bad MC, not a bad something-else. That makes sense. The MC that I bought is this one:

Brake Master Cylinder CONI-SEAL MC114497 fits 87-93 Dodge Dakota

For what it’s worth, the steel body has ‘Made in USA’ cast into it.

I’ve only had one bad booster and that would lock the brake pedal down so you’d have to pull the pedal up with your foot again. I’ve replaced a few MCs and never much paid attention to the rod and never had a leak. So I guess I’d think a faulty MC.

How did you bench bleed the new one? If you aren’t careful, and apply too much force/travel, you can score the seals and ruin the MC. The recommended travel on ones I’ve replaced is 3/4". If you bled it after it was mounted, and used the pedal (and a friend), if the pedal got pushed all the way to the floor, that might have damaged it. Just a thought to keep in mind when you re-replace it.

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Pull the master cylinder away from the booster and inspect the rear for leaking brake fluid. If fluid is leaking get it replaced. And while brake fluid leaking into a booster will cause the booster to fail I can’t imagine that a booster could cause a master cylinder to fail. The push rod will center itself in the dimple on the MC ram. The only critical issue with master cylinder to booster mounting is insuring a slight free play at the push rod.

I read up on bleeding a master cylinder before bleeding the new one and used short strokes. At this point I am going to spring for a booster/MC combo from O’Reilly’s: BrakeBest Brakes - Loaded Brake Booster

The title says “Loaded Brake Booster”. I wonder if this refers to the booster being “loaded” with an MC, or the MC being “loaded” with brake fluid?

I looked at the site and it appears to be as an assembly and ready to bolt in, so I assume that’s what “loaded” means.

Who knows what lurks in the minds of marketing types… :stuck_out_tongue_closed_eyes:

I don’t think I’d trust a MC from O’Reilly and I for sure don’t think you need a booster, but do as you will. If you got the other MC from the O boys, that may be why it leaks.

Did as you suggested, RK. I pulled the MC and there was absolutely, positively NO evidence of a leak from the rear of the MC.

Upon further investigation, after wiping the MC and surrounding area clean and dry, and applying the brakes several times in succession with light pressure, allowing pedal to sink as far as it wanted to, this is what I discovered:

  1. The pedal never went all the way to the floor and I never lost braking power. After slowly sinking about an inch or so after the brakes pads and shoes engaged, the pedal would stop sinking and hold.

  2. The fluid is leaking from the the caps on top of the reservoir. Yesterday when I applied the brakes in the manner described above, in an attempt to locate a leak, I noticed fluid on the driveway directly under the master cylinder. When I opened the hood to find the source the first thing I saw was an active drip from the bottom of the booster. At that point I thought the leak might be coming from the back of the new MC and dripping down the booster. But after today it seems certain that the leak is coming from the rez caps (caps were tight). The top of the rez was wet with fluid after 5 or 7 times pushing the brake pedal to max sinkage. This happened directly after I wiped everything down.

I should mention that the rez is about 1/2 filled.

I am now off to google search for master brake cylinder caps leak. I am certain that some of you probably know why this happens.

There should be a rubber boot under the cover. What was on the old one?

There are two fill caps on the plastic rez. Both have the boots in place. This is a new, not reman MC. This is what I bought: Brake Master Cylinder CONI-SEAL MC114497

The old was the one that came stock on the 87 Dak. Bought it new.