Service brake failure

[FLU419 in VT]

Need some help trying to diagnose a complete brake failure a couple of weeks ago that I thought was a burst line or hose, but isn’t. I tried searching for a similar post, but couldn’t find one, so I apologize ahead of time if this was discussed elsewhere

Temps were cold, and had been for a while, and I think were approximately just a couple degrees above 0 F. Upon starting out, the brake pedal was very stiff, but it would still stop with a bit of additional effort on the pedal, sometimes stopping straight, sometimes pulling a bit to the side. After about half a mile, I had a normal brake pedal, and it didn’t pull, and stopped properly. Another mile into the drive, I started slowing down for a traffic light, and my foot went to the floor, and I had zero service brakes, but still had my emergency brake. Air pressure was good, and the alcohol tank was full, and turned on (I haven’t verified that anything was coming out though), and pumping the brakes did not give any pressure. There were no signs of brake fluid coming out anywhere, and there’s no visible rust on any of the lines that I could see. I didn’t have time to investigate, so I parked it, and went back today, 2 weeks later to figure out what the problem was. There were no signs of leaks, the brakes worked perfectly, and the fluid reservoir was full. I drove it around the lot, putting about a mile on it, using the brakes both soft and hard, and they performed as designed. Holding the pedal down , the brake indicator moved to 70(?) psi, and stayed there for as long as I held my foot down. Any idea what happened? Did I get an ice blockage in the air lines? If so, wouldn’t I still have some brakes, as doesn’t the air system just act as a booster, or would that cause a total failure like I experienced? I’ve driven a 419 that not only had air problems, but also had a hole in a brake line, and even though the pedal went to the floor, there was a very small spot close to the floor where it actually had brakes.

Temps today were in the mid to upper 30s, so there wasn’t any chance of ice in the air lines.

If it makes a difference, I purchased it at the end of December, and drove it from Northern NJ to Northern VT without issue. I’ve been slowly going through it and changing all of the fluids, but have not yet changed the brake and clutch fluids. It’s been driven about 700 miles since I purchased it, and has been essentially flawless, other than this “minor” issue.

Thanks for any insight!

 

[tennmogger]

Your guess about freezing is probably correct. My guess is a master cylinder with some water in it. The 419 is a dual brake system with front and rear circuits separate. As you postulated, the boost system is ahead of the master cylinder and is probably fine. The booster simply assists in making the shaft from your foot push easier so the problem is past the booster. A failed booster would not let the pedal go to the floor.

If the master cylinder were good, for the pedal to go to the floor both the front and rear brake circuits must show no resistance to fluid movement. Chances of that are nil without leakage unless both circuits failed at the same time (and started working again at the same time. Logically that didn't happen.

For both circuits inside the master cylinder to fail, both seals had to fail to push fluid at the same time, yet work again afterward. My guess is water in both circuits in the master, which froze and prevented the seals from pushing any fluid. Leakage past both circuit pistons would let the pedal go to the floor. When the ice melted it would work again. Bleeding the master and thoroughly flushing the system would fix it. Since you are flushing completely, you can put in whatever fluid you choose. I put in DOT 3, the cheap stuff, knowing these systems should be flushed every few years.

If your SEE has DOT 5 in it, in which water is immiscible, the raw water could easily freeze.

Having just worked on a long-stored FLU-419, the reservoirs lids had deteriorated and rain had gotten into the reservoirs and was passed to the master. A lesson learned: be super careful removing and gently tightening all the bleed screws.