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.
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