Yikes, all 4 wheels were smoking? The two times (different cars) when I've lost my automobile brakes on a long, steep mountain road, only one of the wheels smoked. I know the smell of burning brake fluid very well. Looks like this Sherlock was a particularly tough one to solve.
I can't believe that response never occurred to me ..... And coincidentally, I just completed a Bathroom renovation at home that included re-routing plumbing lines. My lovely wife DID use that very line on me, and I STILL missed the response ....
I still agree that boat trailers are not maintenance free.
Some automotive brake systems use a flexible membrane between the fluid and the air vent, so air is not in contact with the fluid all the time. If water were to enter the vent of a system like this, it would not necessarily get into the brake fluid.
I understand your point that as an owner you would take better care of your stuff thanothers. Nine times out of ten, I would perform required maintenance on items that I own, but sometimes, it is better to have someone who has more experience and training work on an issue.
Thanks for all the comments. Here is more information that may help understanding some of the challenges I faced.
All the master cylinders that I inspected have an air vent to the fluid reservoir, usually in the fill cap. This allows the fluid level in the reservoir to fluctuate to accommodate brake pad wear, thermal expansion, etc. However, many (all?) also have a diaphragm under the cap to keep out moisture from the atmosphere, rain, etc. In my case, I don't really know if this is sufficient to keep water out when the actuator is submerged during launching (under a few inches of water for a few minutes). This alone is good reason for me to flush the brakes regularly (which I do).
I don't think water will enter the system from the piston seals at the brake calipers, as those seals are designed to withstand the relatively high pressure of the hydraulic fluid when braking. The static water pressure against the seals from the outside when submerged is quite low, maybe 3 psi (6 ft. / 33 ft. X 14.7 psi). Also, the pistons don't move when under water because the brakes are inactive when launching (see below).
Why the Actuator Gets Submerged.
Contrary to the design of my trailer, the actuators for most boat trailers are not submerged during launching because the coupler remains attached to the hitch when launching. The actuator doesn't get into the water without the back of the tow vehicle being submerged (not a good thing).
My trailer is unusual because it carries a sailboat with a fixed keel. With the keel, the boat sits high on the trailer, so the trailer has to go relatively deep in the water before the boat will float off the trailer. To do this, the trailer has a separate extending tongue and coupler that is used only for launching.
Before launching, I completely unhitch the trailer from the tow vehicle, extend the launching tongue on the trailer, then hitch the tow vehicle to the coupler on the extended tongue. Since the surge brake actuator stays with the trailer frame, it is quite a few feet behind the tow vehicle when launching. Therefore my actuator gets submerged when launching, even though the back of my tow vehicle is out of the water.
Since launching requires use of the launching tongue, the brakes are totally inactive during launching because the coupler at the actuator is disconnected from the hitch. However, this is not a problem since the trailer brakes are not needed at the slow speeds used when launching.
Using A Professional
Why didn't I get a professional to do the repair? In this situation I didn't have much choice. I couldn't take the trailer a repair shop because I couldn't tow the trailer with the brakes dragging. A tow truck wouldn't have helped because the problem was the trailer, not the tow vehicle. Further, the trailer is too long to fit on a typical flatbed tow truck, so it would have required an expensive tractor trailer rig for moving large equipment. There also could be possible bridge clearance issues because the height of the boat and trailer even on the ground is already 12 feet high. And I couldn't take the boat off the trailer because I couldn't get the trailer to water. Catch-22!
On the scary side, one trailer repair shop that I talked to suggested that my best option might be to just tow my trailer to the shop by disabling the brakes. When I asked if he was kidding, he said I would be surprised at the number of boat trailers that do not have functioning brakes. Corrosion, maintenance and repairs can be such a headache (as I have found out), many people just disconnect the brakes. I now give a wide clearance to any boat trailers I see on the road!
The best system for surge brakes would be a solenoid valve that enabled them whenever the tow vehicles brakes were activated. That would remove the backing problem and the downhill problem as well.
I would have suspected corrosion someplace as a first candidate, since it seems that many designs of disk brakes are not even designed to be water resistant. At least, that was my experience with several of the 1970s Plymouth and Dodge products. The caliper pistons would rust ijnside, and the steel-on-steel sliding surfaces outside would rust, and so the caplipers would engage but not release. ON a boat trailer both the calipers and the master cylinder would automatically be suspects. And why would anybody make a brake system out of materials that everybody knows wil corrode quickly?
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