Single Brake Locking Up On Hydrastar Disc Brake Kit After Sitting Several Years

Updated 07/29/2026 | Published 07/18/2026

Question:

A couple of years ago we purchased a disc brake conversion kit for our triple axle trailer w/ 7000 axles. We got them installed, bled and things seemed okay, but our travel plans for that year fell through. So, this year during a shake-down drive I notice one of the brakes was locking up. To clarify, these brakes may have 5 five miles on them. I checked for kinked lines that could allow brake fluid to travel one way under braking and found no kinks. Also, the brake releases as soon as I let up on the brake, while the kinked line scenario above would cause the brake to stay locked up after releasing pressure on the pedal. I even considered perhaps the other five brake lines arent completely free of air, while the locking brake may be locking because it is the only air free line. The other five do activate and the trailer stops much, much better than it ever has, causing me to doubt only one brake line is air free. The other five discs do brake hard. Getting to my questions 1: do you have any other ideas, tips, or tricks I could try or 2: is it possible to purchase only one brake unit?

asked by: Bennett B

Expert Reply:

Hi Bennett, sorry to hear that! Since the brake releases as soon as you let off the pedal, I agree it doesn't sound like a kinked brake line or trapped hydraulic pressure. Since the other five brakes are working as intended, air in the system is also unlikely. What I would do is check the caliper. A sticking piston or slide pins can cause one brake to apply much harder than the rest.

Inspect the caliper to make sure it slides freely and that the pads are installed correctly. If the caliper turns out to be the problem, you don't need to replace the entire brake assembly. You can replace just the caliper with the Hydrastar Replacement Disc Brake Caliper # HYD99ZR, which includes new brake pads and mounting hardware as well.

Where are you headed once you get the brakes fixed?

expert reply by: Robin H

Replacement Caliper
(click to enlarge)

Bennett

7/25/2026

Thank you, Robin, for the response. I have more information regarding this brake issue. Double checking which brakes were doing what, I had my wife video from the sidewalk as I applied the brakes. We found that the left and right front brakes were locking up, not just one. Back in the driveway, after unhooking and leveling, I jacked up the two rear axles. I had my wife step on the brake in the truck while I tried to spin the four wheels. Yep, I was able to spin all four. It seems apparent that the front two brakes work while the other four do not. It seems that my belief all wheels were doing something (in the earlier post) was based on the front two discs were doing a much better job of slowing the rig than all six electric brakes were able to do. I am really looking forward to having all six discs in operation! I have disconnected the lines after the front brakes thinking there may be some crud in the lines, blown them clear, reconnected, and re-bled the system. That did not fix the issue. My ego is on the line here, my last ditch effort may be to take this to a repair shop. Please, no. Any insights or suggestions will be greatly appreciated.

Robin H.

7/25/2026

@Bennett that actually narrows it down quite a bit. Since the front axle is getting full hydraulic pressure and both rear axles aren't, I really don't think you're dealing with a caliper issue anymore. Something is preventing fluid from reaching the rear two axles, whether that's a restriction, an issue with the tee or fittings after the front axle, or the brake line routing itself. Since you've already blown out and re-bled the rear lines, I'd take a close look at the plumbing immediately after the front axle where the line continues to the rear two axles. I think that's where you'll find the problem.

Bennett

7/25/2026

Robin, As noted, I did blow out the brake lines leading from the front axle tee to the middle axle tee on both sides (two lines). I have yet to clear the middle axle tees. We are in agreement, the issue seems like a restriction at both middle axle tees. Since the left and right lines separate before the front axle, if there is any blockage there has to be a blockage on the left side and another one on the right. I'll crawl back under there tomorrow, disconnect the lines between the middle and rear axle on both sides, and blow out the tee. I'll blow out the line running to the rear axle while I'm at it. Thank you so much for the direction and moral support!

Robin H.

7/27/2026

@Bennett you're welcome, hopefully that does it and you can move on. If not, taking it to a shop isn't a bad idea just to get another set of eyes on it before you replace larger parts.