I was driving my 2011 R55 yesterday in NYC in degree weather, about a 40 minute drive and stuck in traffic the whole way so never really moving much. The car was making some extra noise / seemed like it was struggling when under 2k rpms, which I attributed to an aging AC compressor (119k miles)

Shortly thereafter, the brakes began to feel a bit spongy until they got worse with every depression of the brake pedal, until I more or less had no brakes and the pedal went to the floor.

I narrowly made it back to a parking spot, and checked the lines but no sign of leakage. It did however take almost of a 12oz bottle of DOT3 from a corner store.

Has the heat/age somehow caused the master cylinder to fail, and there is a slight leak up there that I can't see? Was the noise I was hearing actually something in the brake system nearing failure?

More importantly, how do I get this thing home/shop? Can I just keep filling it up with brake fluid as I go?

Any info would be appreciated!

 

Getting a car to go is the more fun part. Getting the car to stop is the most important part.
Get the brakes fixed. Sounds like you might be a novias with a wrench. Take to the shop and have the brakes done.

 

If it took that much fluid with out signs of leaking, could be the pads are worn right down. I suggest getting the car in the air, pulling the wheels, and having a look at the brake pads. If you don't have the capacity for that, get it checked out at a shop ASAP.

 

Thanks @njaremka I'll do this. Without having pulled the wheels yet, the pads I can see on the outside are probably about 50%. Anecdotally, I haven't had the issue since I added the fluid; I topped it off and drove it back up the parkway with no incident. Have only driven it around town since, but no apparent issues.

I'm not sure what would cause it to start with normal brakes, lose fluid over a 45 minute period with no signs of leaks, and go back to normal once it's topped off. What I'm trying to say is that it was totally fine up until that point, and wasn't gradually getting worse which is what I would expect as brake fluid goes down as the pads get thinner. Moreover, it seems back to normal now and is not losing fluid with normal driving and braking in normal driving conditions.

Could it have been something with the ambient temperature and perhaps old brake fluid? ie, could it have gotten to a boiling point being stuck on searing blacktop breathing in other cars exhaust, and become quite thin and leaked past seals? It was stuck in stop and go traffic on 100 degree day for close to an hour with who-knows-how-old brake fluid.

Thanks!

 

I would be skeptical that brake fluid, in a sealed system, would boil away at the rate of 12 ounces during a drive. DOT 3 fluid, which is the lowest, has a boiling point of over 400 degrees. You'd need to maintain that temperature for a long while to release that amount of liquid in to a gaseous state.

I'd expect that somewhere, in the system, a seal was compromised and the fluid was pushed out. You can't lose that amount of fluid in any other manner. And eventually it will become evident. If you can't perform yourself, take the car to a pro and have the system checked/pressure tested.

 

I think you're probably right, seems like a problem rearing it's head which exacerbated by excessive brake use, but will inevitably get worse. Maybe it's been bleeding into vacuum booster which is why I can't find any leaks?

Will get it checked out and report back.