Motor Oil In Master Cylinder

Someone (NOT ME) accidentally put motor oil in my son's '66 Chevy
with manual brakes. We fixed a leaky line, but are unable to get
any response from bleeding. Won't bleed at the master cylinder
either. Oil was removed & replaced with brake fluid.

Is master cylinder shot?


Thanks,
Glenn F.

More then likely oil damaged the seals inside cylinder.

I think I would rinse it out good with brake fluid, then bleed it. Then if you can't get it to work, get another master cylinder.

Not shot. but must have all new rubber components (cheap). All other rubber components must be replaced as well. flex hoses to the front brakes, and rear rubber hose to the differential, and all wheel cylinders need to be rebuilt. All internal surfaces must be purged (alcohol followed by air followed by Brake fluid. Jim

My son put power steering fluid in the master cylinder reservoir of my '88 Chevy pickup. I ended up replacing:

Master cylinder
Rear wheel cylinders
ABS unloader valve
Proportioning valve

As I recall, the parts bill was close to 500 bucks. I also disassembled the front brake calipers and cleaned them out good. (The calipers were brand-new, so I didn't want to replace them.)

At the very least, your master cylinder is trashed.

Back when I was taking Auto mechanics, the teacher passed around several glass jars with rubber parts in them, in different kinds of fluids. Rubber parts do strange things when immersed in the wrong fluid they aren't developed to handle. Some of them swell great big.
To be safe, I would replace the master cylinder, and wheel cylinders and flush all the lines with plenty of brake fluid.

If I remember right motor makes rubber brake parts get soft and swell up.

That's a bad thing to deal with. It's one of those "pay me now or pay me later" situations.

Best to replace every rubber component, including the hoses. The lines must be flushed thoroughly. Any trace of oil will come back to haunt you!

Unless the wheel cylinders and master cylinder have been replaced, real good chance they are rust pitted. Be a good time to just replace them. Not much price difference if you consider your time spent cleaning and honing.

Now would be a good time to convert it to DOT 5 silicone brake fluid. It will work well in any application, but it is especially good in vehicles that do a lot of sitting. It does not absorb moisture like glycol fluids do. But you have to remember it's in there, because it is not compatible with other fluids. I recently worked on a 20 year old Harley that had DOT 5 in the brake system, that fluid was crystal clear, everything looked like new!

I traded a '74 Chevy 3/4 ton to a firewood guy for an expected 2 cords of firewood. It had a real leaky power steering assembly. Firewood = dumbaxx around here (except for a few), so he put the only thing he had in the steering reservoir - brake fluid. Stupid is lucky some times, swelled up the seals and he ran the truck for a couple more years - STILL owes me some firewood. Saw him a few days ago and of course denied all responsibility - JERK ! P.S. - glad to be rid of the truck, it was mostly toast. ;^)

Dot 5 does not work well in any application and may/will void the warranty if used. Here I set with 7 gals and won't sell it, its not what its made out to be... BTDT and flushed that chit out of the system.

Dot 4 fluid is the way to go.

Hmmmm...not a good report. How can motor oil be so damaging, causing rust pits, etc.? It's sure a lot gentler on paint, etc. than brake fluid.

Thanks,
Glenn F.

Hobo, got me curious as to what problem are associated with DOT 5.

Did some looking around, there are some problems!

Mostly, it does not work well with anti lock brakes, and is not intended for racing or high performance/high heat applications. The other concerns are it requires a completely clean and dry system, and care must be taken to not aerate the fluid. And there is the possibility of it getting the wrong fluid added to it, which I don't know what that would do...

But, I'm still hearing good reports on using it for antique vehicles, RV's, surge brakes, anything that does a lot of sitting. Not supposed to grow the black grunge that settles to the bottom of the cylinders.

"More than likely" how about 99.99% that it surely damaged the seals or cups in the master cylinder and beyond. "Oil was removed" was that done by disassembling the master cylinder and cleaning all the parts with brake clean or alcohol within 15 min. of the oil being poured in? If not the rubber parts are to damaged to be reused. Any wheel you opened the bleeder on most likely needs the rubber parts replaced. If by chance minimal fluid came out of the rear bleeders (teaspoon of less). You may get by with blowing back through the lines of both rear wheels and flushing with brake clean or alcohol. On a system like that of you had a front bleeder open much at all enough fluid flowed by gravity to contaminate the front hoses and wheel cylinder rubber parts.

The brake components are made of rubber. Rubber (not to be confused with Neoprene, Nitrile, or Buna) is not compatible with oil or any petroleum products. It will swell, loose it's ability to seal, and cause the pistons to bind in the cylinders. The binding will cause the shoes to not retract, and the master cylinder not to retract far enough to open the bypass port, resulting in the fluid building pressure, and the brakes failing to release, and over heating. Same thing with the hoses, they will swell internally, blocking or restricting the flow to and from the wheel cylinders.

The rust pits are not caused by the oil. Chances are, at the age of the vehicle, the cylinders were long ago beginning to rust pit from moisture in the system, a very common problem in older vehicles.

So, realistically what's the best thing to do here. Oil was put in the master cylinder about a month ago. Vehicle has sat ever since, only being driven in/out of the shop about seven times. Oil was extracted from master cylinder with a bulb syringe. Only left rear bleeder was opened. Less than a teaspoon of what looked like brake fluid came out. Most of the Moto dr oil leaked out the leaky line in the month the truck has been sitting.

Thanks,
Glenn F.

Sorry, there is no shortcut.

You have to replace all the rubber components and flush the oil out if you want to have reliable brakes.

It doesn't matter if the vehicle was driven, it's a chemical reaction.

The good news is, brake components for that model are cheap, the system is simple to service. If the system was leaking before, it was in need of service anyway. Now you only have the inconvenience of having to flush the lines!

How do I flush out the lines?

Glenn F.

> How do I flush out the lines?

Assuming the vehicle hasn't been retrofitted with dual brakes, it's pretty simple: Bench-bleed the new master cylinder before you install it. Then bleed each of the wheel cylinders, either before or after you replace/rebuild them. (I'd bleed the system before replacing the wheel cylinders.) Once you've pumped enough fresh fluid through the system so you're getting clean fluid out, it should be OK.

If the vehicle has dual brakes, the proportioning valve should be replaced in addition to the master and wheel cylinders.

> Stupid is lucky some times, swelled up the seals and he ran the truck for a couple more years

Hah! Some folks can just coast through life, not realizing how dumb/lucky they are.

Steve,
I have 3 DOT5 vehicles with over 90 years of use.
No problems yet, but I changed every thing except some hard lines when I started using DOT 5.

The standard practice in the industry is everything rubber that has contact with the fluid must be replaced after a complete flush of all lines at the wheel brakes.

Water will get in it when it does it puddles in one spot that spot will rust. It can not stand up to heat under the hood well. I have seen it expand and cause the brakes to drag. In a clutch hydraulic system I have seen it heat up and cause the clutch to slip.

I had a issue with a 67 442 the guy had replaced all the lines with SS and went with SS lined brake calipers along with Dot 5. The calipers had a lifetime warranty and were leaking when taken apart all of them had bad pitted calipers. The manufacture rejected the warranty and stated dot 5 was the root cause. They said go back and read the warranty the use of dot 5 will void the warranty.

You could see were one drop of water in one place in the calipers started eating away at the liners. If you use dot 5 you still need to flush it every 2/3 years just like you should do with dot 3.

The problem I have seen with dot 3 is there is so many cheap brands out there that are not compatible with the rubber parts in the system. I use name brand dot 4 and never had a issue with it.

American manufactures are catching on to adding brake fluid flushes in there maintenance schedule, its been the norm for European and most Asian manufacturers to recommend the fluid be serviced.

The next problem is the black chit were does it come from. Its a reaction with something in the system.

http://changebrakefluid.blogspot.com/p/brake-lines.html

If you want to see a real mess get a hold of a system were dot 3 and 5 have been mixed :shock: I did the maintenance for the local USPS for 20 years, They used dot 5 I never seen the benefit in fact I thought it made it worst. They put a big dot 5 sticker on the top of the lid on the master do you think a normal guy knows what that meant :lol:

You guys with chebby pick ups that have rear disc brakes with aluminum calipers, calipers are in your future just wait till you see that black mess that comes out of them. Once you get it replacing the calipers and a good flush is the only answer.

You need to use alcohol or brake cleaner to flush out the oil. An air conditioning flush can would work good or just a couple cans of brake kleen. Just flushing brake fluid thru will not suffice.

I have an older 2 ton dump truck that I completely rebuilt the brakes on in 1993, new master cylinder, wheel cylinders, tubing, hoses and hydrovac. I checked with all of the manufacturers of the wheel cylinders and hydrovac and they all approved the use of Dot 5 fluid. The truck sits a lot, but gets used periodically and has never had any problems with the braking system since then. I would definitely use Dot 5 fluid again for this type of application. As long as the system is not open to the atmosphere, I don't see how it is going to absorb water. I have not flushed my system during this time, and based on my experience with servicing other vehicles using Dot 5, it would be a waste of time and money. It would be a mistake in my opinion to use Dot 5 fluid if not starting with a totally clean system, and without verifying that all of the components are compatible with silicone. Also, as stated, Dot 5 is not compatible with ABS systems.

Replace the wheel cyl,the master cyl and all three brake lines. I saw an insurance company total a Land Rover over the same thing(ABS).

When I was a small kid, my brother (about 6 years older than me), added engine oil to the master cylinder of our F500 grain truck. We had to have every wheel cylinder and the master cylinder replaced or have a new kit put in each component. My dad knew that he had done it, but didn't really blow his top over it. Chalk it up to a learning experience.
You need to replace all the rubber components.