Can improper rotors and pad replacement (by a repair shop, not me!) blow out the master cylinder?

Hey car talk community -

We have a 2004 VW Passat Wagon, which my wife drives. Recently we had the brakes done at a local Firestone repair shop while getting the tires changed. When they did the job they replaced the rotors and pads on all 4 wheels. When we got the car back the brakes worked, but the we had to press the pedal all the way to the floor to get the brakes to grab. We though that perhaps the brakes just needed to break in, so we didn’t think much of it. We kept driving for a week or so, then noticed the ‘brake pad wearing’ smell coming from the back wheels.

We brought the car back to Firestone to have it checked out, and they are now telling us that we need a new master cylinder, and it is going to cost us $400 to repair. The brakes do need more work, but the thing is, the brakes worked better before they replaced the rotors and pads. We didn’t have to go all the way to the floor to get them to grab. As expected they are denying all accountability for blowing out the master cylinder.

So, my question is this: is it possible to blow out the master cylinder while changing pads and rotors? My thought is yes. The logic I used is this: if the cap was not removed from the master cylinder reservoir when the calipers are compressed the pressure would be sufficient to blow out the master cylinder. BUT I am no mechanic and I know very little about brake systems. And when I say very little, I mean I know nothing.

Does anyone have any thoughts on this?

Thanks for you help!


It sounds like they did not bleed the system properly. If your master cylinder was OK before, it should still be OK. I suspect these guys don’t really know what they are doing and just keep throwing parts at the problem, at your expense.

I have never heard of new brakes “blowing” the master cylinder, unless it was on its last legs to start with.

Hey Docnick -

Are you saying that if they didn’t bleed the system it could blow out the seal in the master cylinder? It has nothing to do with compressing the calipers?

They offered to replace the master cylinder and only charge for the part, which they say costs $270. Google says it can cost as low as $90 and as much as $145. VW parts dept says $320. I got them down to $245, but I am really hesitant to have them do the work. They warned that the ‘power booster’ may be bad too, and my fear is that they will break that while doing the cylinder and want more money.

Is a master cylinder hard to replace? I am handy with a wrench, but haven’t ever done brakes before…


Even if the cap was not removed and the calipers forced in, the cap is vented to allow air to enter as the fluid level drops.

Since you’ve now had to push the pedal all the way in, however, you may have had to push it beyond its normal range of travel and the piston seal may have become damaged in that manner. The shop may have also done this in the process of bleeding the system. At that point it becomes debatable whether that’s the fault of the shop or the MC was well-worn anyway. You don’t say how many miles are on the car, but at 9 or 10 years old I’m inclined to suspect normal wear & tear on the piston seal. Pushing the piston farther simply brought a well worn MC to its inevitable conclusion.

Sorry, but I have to go with normal wear & tear.

Hey tSMB -

The car has 140k miles on it, and as far as I know the MC has never been replaced. Good to know that it is possible that this happened due to normal W&T.

Great info, thanks!

Going a little further with this, there is always the possibility of doubt and that is what a parts pusher mechanic can thrive on. With that in mind, there is a remote possibility that there may have been a corroded area deep into the stroke of the master cylinder that remained untouched until the brakes were worked on which would require bleeding which would require the entire stroke of the master cylinder to be used. If corrosion was extensive enough, it could conceivably but not likely damage the lip seal on the rubber pistons. That said, I have never had that problem and it is unlikely with a 2004; too new.

If the power booster works as intended, then saying that it “might” be bad makes me wonder about the honesty and ability of the mechanic. You should ask the mechanic what makes him think that the working booster might be bad and then be sure to listen for fast talk Ask what “bad” means. Bad is not a technical term that tells anything specific…

You raise a good point about the power booster. I almost missed that comment.

If a power booster is bad, it’ll typically make the brakes feel “hard”. It will not cause them to go full-length to the bottom of the stroke.

It’s very easy to “quick test” the power booster. With the engine off, push the brake pedal repeatedly until it feel hard. That will purge any resuidual vacuum from the booster canister. Then, while pushing on the brake pedal, turn the engine on. You should feel the pedal drop a bit and the “feel” soften. That’ll be the vacuum taking effect.

@WhaWho, when a master cylinder is leaking externally, there is a chance that some of the brake fluid will wind up in the booster and possibly damage the diaphragm.
Technically a booster that has had its diaphragm exposed to brake fluid in this way should be replaced.
That said, I’ve replaced many leaking brake master cylinders, and occasionally I have seen that some of the brake fluid did find its way into the booster.
If the booster operation was normal before the master cylinder replacement, I would wipe out the booster as best I could. I have wiped out several boosters over the years and they’ve all survived.
What I mean to say is that I would work on the cars months or years later and the booster was still working.
I believe Firestone said the booster might be bad because they wanted to inform you what the worst possible outcome could theoretically be.
They don’t want a scenario where they agree to replace the master cylinder without mentioning the booster, only to have to call you later to inform you that the booster is also bad.
Best to put all the cards on the table up front.
That’s what they’re doing.

Mountainbike called it. They didn’t bleed them properly. What happens is that you push the brake pedal farther than it usually goes, which pushes the rubber-edged piston in the MC farther than it usually goes. It then picks up all the gunk it’s been depositing at the end of its travel over the years. That gunk sits between the rubber and the cylinder wall, making it impossible to get a good seal.

In my opinion, you have an old part and should not expect the new one for free, however, your part did die prematurely due to their incompetence (which is not surprising - it is not a 100% rule, but good mechanics generally aren’t found at chain stores).

Additionally, they let the vehicle leave even though you had to push the brake pedal all the way to the floor to stop - that’s dangerous as hell, and was gross negligence on their part.

Of course, that’s offset by the fact that you screwed up by not turning right back around and telling them the brakes were messed up, before you even got out of their parking lot.

All in all, I’d say a 50/50 split for the cost of the repair would be fair - but that’s if you feel like trusting Firestone again. Personally, I wouldn’t, especially since these guys have followed the chain-store stereotype by performing incompetently and negligently.

I also think whoever you take it to needs to look at your brake lines, because smelling “wearing pads” (which I take to mean hot brakes) indicates a sticking caliper, and a bad MC that can’t build pressure should cause exactly the opposite. That suggests that one of your lines might have collapsed internally, or have a blockage (perhaps from some gunk tossed into the system by the MC) that is preventing fluid from backing out of the caliper to release it when you aren’t braking.

Good points, Shadow. I agree that a new shop and a total system look-see is worth the additional cost rather than returning to Firestone. There are one too many “red flags” here.

I would also strongly recommend finding a well regarded independent shop to look into the situation. The situation looks very suspicious.

So far, no mention of ABS. If this car has ABS and the calipers were compressed with the bleed valves closed and /or the bleeding procedures for a vehicle with ABS were not followed, then the problem might be in the ABS and not the master cylinder. Any thoughts?

Depends on the car. My '07 TL with ABS does not require any special procedures related to ABS during bleeding. You bleed it like any normal non-ABS car. You’d have to check the shop manual.

I pulled the ABS codes with my VAG and nothing came up, and the ABS lite is not lit.

I also think Mountainbike is on point. That’s a very plausible thing to have happened.
Firestone is not the place you want to mess with your car, imo.

I am 100% with RemcoW. My employer took a vehicle with a leaking valve cover gasket and dried out CV joints… Firestone wanted almost 5,000 for the repairs… CV axles 2 hours, and $90 for the brand new OEM. Valve cover gasket, 30 minutes and $12 for the gasket… So at $100 an hour that would be a TOTAL of maybe $400… Firestone said it was a leaking head gasket and wanted 1800 for that and then 2400 for the CV axles… Ya, DO NOT trust them!

If Firestone replaced rotors and pads, I am assuming you have drums in the rear and that smell is the E-brake (handbrake) shoes most likely not adjusted when replaced and as a result are stuck to your rotor. Test this? Just lift the rear of the vehicle, both wheels, put it in neutral (for Rear Wheal drive) and try to turn a tire with the Hand brake off… If the tires don’t spin freely with only a light SOUND of scraping, they need adjustment and FS ******* up.

Brandon , this thread is 6 years old. And Remco has not been here in 5 years.