Hopefully this will be helpful to others out there with the UFP DB-35 disc brakes on their boat trailers. Mine is a 2006 Boatmate trailer with tandem axles and 4 disc brakes. A few years ago I had a caliper freeze up and rather than worry with it mid season I just removed the pads and rolled on with 3 working brakes. Well, being one to procrastinate sometimes, I rolled like that for a couple years. This winter I was driving my truck one day (not towing) and when I came to a stop heard a metallic grinding coming from the front. When I got out to inspect, I found the passenger front rotor was damaged and the brake pads gone on that wheel. This surprised me because I had just done the brakes on the front about 35k prior. The original pads had made it probably 75k or so. Upon disassembly and repair I found the drivers side pads almost down to the metal as well and the rears also with only maybe a 1/4 inch of pad left! Now I was curious as to what had caused the super fast wear of the front pads and my only thought was to check the trailer brakes. On to the boat trailer....
I knew about the one bad one but when I checked, I found 2 other calipers that were either seized or partially seized so I guess I had been towing basically with no assisted braking from my trailer for quite a while. I could not remove the frozen piston from the original bad caliper no matter what I tried so I ended up ordering a brand new caliper with pads included for that one. The other brake on that side seemed to be ok....I was able to take it off and work the piston in and out. The pads were fine so I cleaned it up, greased the slide pins and pad contact points and re-installed. When I got to the other side of trailer both of those calipers had seized pistons but I was able to remove them. Looking at saving as much money as possible I opted to order new seals and pistons and rebuild the two bad calipers. The entire caliper assembly ran me around $80 for the first one but I was able to find 2 pistons and two seal kits for around $55 so I went that route for the other two. I found that you can either order the pistons by searching for UFP DB-35 or you can source out pistons and seals from a 2000 model Kia Sephia believe it or not. I tried to order the UFP stuff at first but after having some delays on stuff being back-ordered , was able to just pick up the Kia parts and it was cheaper anyway.
Its pretty easy to remove the caliper from the trailer. Unfortunately I did not take pics but you basically safely lift the trailer, remove tire/wheel and then remove the two bolts holding the caliper to the trailer hub. You will need to remove the brake line as well...simply loosen the banjo bolt. Be ready with a catch pan under the caliper to catch the brake fluid that will leak out. Once the caliper is removed take it to your work area and remove the retaining clip by popping out with a screw driver. I used a screw driver to kinda wedge against the pads applying some pressure to make a little space between them and the rotor. You should then be able to remove the front and rear pads. Then you can remove the caliper from the mounting bracket. I was able to get the old piston out of the bore my using compressed air into the brake fluid port on the back of the caliper. You want to be very careful when doing this as it does not take much pressure to force out the piston! Make sure to place a block of wood or something between the piston and the caliper and obviously keep body parts out of the way. There are plenty of Youtube videos that show this process.
Once the old piston is out check it for damage. My old ones had surface rust and I was able to clean that away but there was pitting that I could not remove. You can use a screwdriver or small pick to pull out the square cut seal from the piston bore and you can remove the dust boot easily by hand. I ordered the pistons and seals/dust boots from carparts.com. Search on 2000 Kia Sephia as previously mentioned. Use the time waiting on the new parts to clean up your caliper and bracket. I cleaned the seal grooves in the piston bore with my dremel and wire brush to remove the rust and crap that had built up. I also cleaned the brake pad slide points and the slide pins on the caliper bracket. I used Sil Glyde brake lube to grease up the slide pins and the brake pad stainless slide points. When the parts arrive use clean brake fluid to lubricate the square seal and pop it back into its groove in the piston bore...very easy. The dust boot is a little more tricky. I found the best way for me was to lube it up well with the Sil Glyde and slide it over the piston. I pulled it all the way down over the piston until the bottom flange that goes into the bore is hanging off the bottom a little. I then held the piston over the bore and kind of worked it in to place while slowly inserting the piston. It is hard to describe but it will make sense when you are doing it. It is obvious when it is correctly seated. Then just use your hands to press piston fully in to bore while making sure it is straight. When its lined up it should go in without too much trouble. When bottomed out the top of dust boot will snap into a groove on the top of the piston. I then used the compressed air trick again to ease the piston back out to make sure dust boot was sealed properly and that it all was working as it should. Then its just a matter of putting the caliper and bracket back together and re-installing the pads. I reused all the old pads and hardware on mine because they had plenty of wear left.
Slap the rebuilt calipers back on your trailer...torque bolts to 55ft/lbs with some blue loc-tite and re-install the brake lines. The only thing left to do is to bleed the brakes! Hopefully this helps someone save a little money. I have included some pics of various stages of the process...not really sure how to put them in any kind of order.
Caliper bracket with slide pins and shims for pads ready to be lubed
Ready to be put back together with caliper
This pic shows how I got the dust boot started while inserting piston
dust boot with some lube before being installed over piston
Dust boot and piston seated in caliper bore
Pic showing caliper bore with new square seal installed
Pic of square seal before install in to caliper bore
Pic of how I situated dust boot on piston prior to install...worked best for me
Shows where to apply compressed air to get piston to move out of bore
Just another shot of completed assembly of piston and dust boot