Bleeding trailer brakes
 

I am not sure what I am doing wrong. The disc brakes on my tandem axle trailer were no longer working. After checking for leaks and not finding any and the brake fluid reservoir was full, I assumed I likely had a bad master cylinder/coupler or a bad reverse switch. I tested the switch and it appeared to be working so I ordered a new coupler, had it painted to match and installed it. New coupler also has a new reverse switch.

I was bleeding the brakes (which I have done 20 times before on other vehicles/trailers) by not plugging in the trailer (reverse switch), "pumping" the master cylinder (with my truck) with the trailer wheels blocked, holding pressure, running to the back (starting with the furthest wheel from the coupler) and making sure I never ran low on fluid. I recognized I likely needed to bleed these lines a lot as I had to pull some of the lines out to replace the coupler plus I had a new coupler which would make sense.

For some reason I couldn't bleed all of the air out of the system. I did this for 4+ hours and couldn't get the air out. I was able to get some pressure on the brake lines but not enough for the brakes to really stop the trailer. I made sure I never ran out of fluid in the reservoir (by filling it up after every bleed) but I simply couldn't get the pressure up.

I had bought a power bleeder before I started this project but the design of the reservoir (no threaded cap - just a rubber plug) made the power bleeder useless.

Any advice? I can't find a leak anywhere in the system - could there be a small leak that doesn't leak oil but lets air in? Maybe I just need to keep going and it will eventually build pressure?

How old is the fluid? If its super old, it is likely full of water which makes the fluid more "compressible" and makes brakes feel "mushy" ultimately making the pads grip the rotors with less force. Brake fluid likes to suck up water out of the air. If your fluid is 10+ years old, this could be the issue. Bleeding brakes on a trailer is a total and complete pain because you can't use a power bleeder as you already found out. All you can do is push on the tongue and pump it that way - old school. If there is a better tool, I'd love to know about it. I'm thinking you just need a flush.

One other theory - strange you have all the air in there. The only time I've had air is when the fluid boils. Is it possible one caliper is stuck in the "on" position and dragging against a rotor thus overheating the fluid? Once you overheat the fluid and get air in there, you also get more water. The only way to fix that is a complete flush. I used to track my cars all the time so I have great experience with boiling fluid - lol. Try jacking each wheel off the ground and making sure it spins freely so you know the brake is not suck on. That will help rule that theory in or out.

Boat is 2008 - I am the second owner but I am pretty confident it has never been changed. This trailer has A LOT of miles on it - not only did I have it driven from Wisconsin to CA - I was doing 240 miles roundtrip on the weekends on the freeway for 5+ years before I bought a lake house. I am sure they have been "hot" a number of times and could have damaged the integrity of the fluid. The fluid in the old reservoir was not clear either.

How do I go about doing a flush? That sounds horrible if I have to just pump it all through. I will spin each wheel as well.

I also need to find out why I am still getting air in the lines or I simply didn't move enough fluid through the system.

Could be air in the lines because you did not move air through the system. Since the fluid is old, it will have loads of water in it which makes it boil very easily. This could very well account for the bubbles. I'm not sure how much fluid the trailer holds. Is it a Boatmate trailer? If so, call them, give them your serial number and they can give you all the trailer specs. I suspect it holds something like 2 quarts of brake fluid but that is a pure guess and I could be completely wrong. Assuming it holds 2 quarts, you would want to flush more than 2 through the trailer. You can either pump all the fluid out so nothing but air is left (tough to get the air out) or combine the fluid. To combine, as you pump some out, add more....pump some out, add more. Doing it this way takes more fluid but you will not get any air in the system. The only way I know how to pump the fluid out is to push the trailer tongue back at the trailer while the trailer can't move. Start at the tire farthest from the mast cylinder and open the caliper. Keep pumping the brakes using the trailer tongue while adding more fluid until the fluid coming out of the caliper looks like new. Again, assuming you have 2 quarts in the trailer, I'd want to get 2 quarts out at the farthest corner and work my to the other tires from there. Boatmate may have a better procedure for doing the flush. Their customer service is great so I'd call them and get the specs of the trailer fluid and how much it holds. Ask them for a recommended flushing procedure. The old manual procedure is the only way I know to do it.

YouTube has some good videos on how to flush trailer brakes. I looked at some recently since I plan to flush the brakes on my tandem disc trailer soon