Will Lisa's big mistake lead to a crash?

Dear Car Talk

Dear Car Talk | Sep 01, 2011

Dear Tom and Ray:

I'm an idiot. I just poured synthetic motor oil into my master cylinder. Am I destined to crash?

-- Lisa

RAY: Well, not unless you drive the car, Lisa. If you leave it parked, you should be fine.

TOM: It's a tricky problem, Lisa. We checked with the brake engineers at Bosch, and they said that the Ethylene Propylene Diene Monomer seals in the brake components are compatible only with glycol-based fluids.

RAY: In other words, you're gonna crash.

TOM: Motor oils, including synthetic ones, are based on mineral oils. The problem is that mineral oils and rubber don't mix well. The oils can cause rubber to swell or deform, and because there are lots of rubber seals in your brake system, that eventually can make your brakes stick, leak or even fail completely.

RAY: So the safest thing to do would be to replace every part of your brake system that uses rubber seals. That's pretty much everything except the steel brake lines! So that's going to run into big money.

TOM: A less-drastic measure (assuming you haven't driven the car and mixed the synthetic oil with the brake fluid) would be to siphon as much as you can out of the brake-fluid reservoir. Hopefully, you'd get the vast majority of it out that way.

RAY: Then you could remove the brake lines from where they connect to the master cylinder and flush out the cylinder -- without sending any of that potentially contaminated brake fluid down the brake lines.

TOM: Or you could just replace the master cylinder entirely, since that's the component whose seals are most likely to be affected by the oil.

RAY: In either case, once you have a clean master cylinder, you could then reconnect the brake lines and flush the rest of the system by bleeding the brakes at all four wheels.

TOM: But remember, you'd be taking a shortcut with your brakes! So have the car towed to a mechanic you really trust, and ask him to try to figure out how much oil has gotten into the brake fluid and how far it's gone. Then you can get his best advice before deciding on a course of action.

RAY: If it were my brother's car, I'd probably take my chances. But if it were a real car, I'd probably go the replacement-part route.

TOM: And if you think you're an idiot now, Lisa, wait 'til you see what phrases come to mind once you see the bill for that job. Good luck!

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