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Master Cylinder Failure - Tow company at fault?

A few weeks ago, my 1992 Honda Accord LX was towed by a local towing company about 10-15 miles due to a broken windshield(couldn’t drive it in this condition and the tow was from an impound lot)and since the car was drivable, the tow truck driver dropped the car in the street and told me to back it into my friends driveway. As I approached my car, I smelled HOT brakes which I asked the driver what was going on with my car. The driver looks in my drivers side window and says: “oops. I forgot to release your parking brake.” I get into my car to start it, stepped on the clutch (5 speed manual trans) and hit the brake - the brake pedal goes to the floor - never happened prior to my auto vs deer accident - I tell the truck driver, I have no brakes. He tells me to pump the pedal, it should come up. I do that and the brakes get better, but are spongy at best. I back the car up and park it. A day later, the windshield got replaced, I picked up my car - still had spongy brake pedal, so I call my auto shop and schedule a brake inspection. This past Monday, they find the brake shoes to be glazed over and smell like they were super heated at one point,but the shoes are still ok, so they proceed to bleed the lines thinking the brake fluid had “boiled”. They replace a cylinder in the back due to a seized bleeder breaking off, then bleed the lines - still spurting air after bleeding the system. They replace the Master Cylinder and re-bleed the lines - problem fixed. My question for all of you is could the superheated brake shoes heat up the environment around them, “boiling” the brake fluid in the cylinders and by pumping the brakes, circulating the hot brake fluid, cause the Master Cylinder to fail internally? There were no external leaks. The tow company has refused to take responsibility for any of the repairs since the shoes were not replaced and according to them, hot brake shoes cannot boil brake fluid. My brakes worked fine until they had towed my car. Just looking for feedback and whether or not the hot brake fluid could cause my Master Cylinder to fail, or if my master cylinder was in need of replacement before this incident occurred. Can I hold the towing company liable for this repair?

Yes 100% super heating the brakes (IE towing with the brake on)could cause the brake fluid to boil. I am just not sure how that heated fluid would damage the master. Brake fluid does not flow like let’s say power steering fluid. It does not get cerculated through the system. However what I think happened was since your car was at an angle nose up, the hot fluid wicked up the brake lines to the master. They should be responcble, but good luck getting money from them. The shop should also look for warped rotors or drums.

I agree. The brake system may need to be totally overhauled. “Oops”? That guy is a moron.
Hate to say it but, If you do end up dealing with their insurance, they may deem it ‘totaled’ because of the age of the car.

Does your shop believe this is what damaged your master cylinder and will they put that in writing? If so, tell the towing company (in person or in a registered letter) that you have a written statement to that effect and that your next stop will be in small claims court unless they make things right.

By the way, I think it’s better to use a flatbed truck when possible. There’s less chance of transmission damage, plus you’ll have less debris thrown at your car by the tow truck’s tires.

If the brakes got that hot, it probably got the fluid boiling, especially if it was old fluid and had some water contamination. This causes air bubbles in the fluid which will rise to the highest point in the system. The air bubbles and hot fluid also expand and put pressure on the seals. It’s conceivable that the master was damaged but also just as likely that it only had a lot of air entrapment. It’s difficult to properly bleed a master without disconnecting the lines and using a “bench bleeding” procedure. By the time you go through all that, it is usually easier and better to just replace an older unit that probably has a lot of crud built up inside of it anyway.

RemcoW No they will not total the car over this. The tow company should pay for all repairs. If I was the adjuster on this I would have the rear brake shoes replaced. Just because of liability.

Oh, I was figuring this sort of claim was the same as an accident claim. No doubt you’re right.
OP: go get 'm.

The claim would be against the tow company’s liability policy, 100% covered. The OP will need to work wth the tow company on this.

Not only could superheated rotors cause the fluid to boil, it can also (and probably did) destroy the elastomer O-rings and seals in the calipers. And the flex lines that connect to the calipers. And, apparently, in the master cylinder as well. I guess the fluid transmits heat better than I would have guessed.

I guess the fluid transmits heat better than I would have guessed.

It’s not just the fluid that’s transmitting the heat…but the brake lines also.


It seems quite a stretch to blame a failed master cylinder on the rear brake shoes dragging. I have repaired brakes that had drug until the hub grease was liquified and the rotor scorched with no damage to the master cylinder. That scenario seems unlikely.

When you push a brake padal to the floor on a vehicle with a 20 year old master cylinder force the piston seals through a normally unused area of the master cylinder bore. Twenty years of dirt and corrosion will damage the master cylinder seals.

BTW brake tubing is so small you can hold in your hand while heating it with a torch and not burn youself. Not that there is any reason to heat it but to illustrate that it won’t conduct heat from the rear of the car.

Nevada, I think you’ve explained it. I myself would not have thought the heat generated by the brakes could damage the master cylinder, but made the assumption based on the facts of the case. You’ve brought up a fact that I missed completely.

A tip of the hat to you.

Rod, The thing that I think makes this case different is the angle of the car. If it was on a wheel lift (which I assume it was), it was probably angled up at around 25-30 degrees. Otherwise I completely agree with you.

I’ll add. With the brakes shoes on both wheels wedged against the drums, as the pressure of the boiling fluid builds the only way for the fluid to go is back through the master cylinder, overwhelming the seals and forcing the fluid out through the master cylinder. If the fluid boiled enough its possible the hot vapor itself was forced past the seals as well.


I am just not sure how that heated fluid would damage the master.

Because when he stepped on the brake pedal, the boiling fluid caused a pressure loss that let the pedal sink to the floor, which moved the plunger beyond its normal travel area. The plunger then picked up all the crap that’s been accumulating on the walls that aren’t normally swept by the plunger, and that crap is now deposited on the plunger, creating an imperfect seal against the wall.

The tow itself didn’t destroy the cylinder, but stepping on the brakes when the fluid was still hot did. And since it was the truck driver who told him it was ok to back the car into the driveway, he acted in good faith on the advice of an automotive professional, and that advice finished the destruction started by the same automotive professional. Or, in other words, it’s still the tow truck driver’s fault.

Beautifully explained, Shadow.
And i agree. The MC should also be covered by the tow company’s liability policy. Honestly, as long as the driver admits his error, I doubt if the insurance would balk at such a relatively small claim. It’s generally the ones that involve bottomless-pit damages (like medical and destroyed engines) that insurance companies tend to balk at.

If OP is remotely handy, master cylinder replacements are easy, and you can get the whole job done for well under $100. I’d rather do that than have a claim on my insurance that will ding my rates for 5 years.

It wouldn;t be on his insurance. It should be covered by the tow company’s insurance under their liability section.

If you’re alluding to the tow company also being a garage and their doing it directly at their cost, you may have a point.

The very overheated brakes in the rear did boil the brake fluid and that is the same as getting “air” in the system. The master cylinder was fine and would have survived if you had waited for the brakes to cool before stepping on the brakes. It would have taken several hours to cool down.

Since the driver left you in the street that forced you to move the car and therefore step on the brake petal. This is when the petal went to the floor and the plunger in the master cylinder moved beyond its normal range and picked up dirt with scored the walls of the master cylinder.

Therefore I believe the tow company is liable for damages since their actions forced the owner to move the car and step on the brakes in the process. Now will the owner get the tow company to own up to the damage? That’s not going to be easy.