How to Convert Your Trailer From Drum to Disc Brakes
The Short Version
A complete drum-to-disc conversion has four parts, not one:
- A disc brake kit — rotors, calipers, brackets, and pads sized to your axle. A slip-over kit is the fast path because it keeps your existing hubs.
- A 1,600 PSI actuator — the part everyone forgets. Drum actuators only make ~1,000 PSI. Disc calipers need 1,600 PSI to clamp.
- Brake lines & fittings to plumb the new calipers.
- Wiring — only if you're also moving from electric drums to an electric-over-hydraulic (EOH) system.
Skip #2 and your new disc brakes will feel weak or won't engage at all. That single mistake causes more conversion headaches than everything else combined.
Why Convert to Disc in the First Place?
Drum brakes work, but they have real limitations on a trailer — especially a heavy or frequently-launched one. Here's what disc buys you:
Shorter Stops
Disc dissipates heat far better than drum, so it resists fade on long grades and repeated stops when you're loaded heavy.
No Adjusting
Disc brakes are self-adjusting. No more crawling under the trailer to re-set drum shoes every season.
Saltwater-Friendly
An open rotor rinses clean and dries fast. Drums trap water and grit inside — a corrosion trap on any boat trailer.
Easier Service
Pads and rotors are quick to inspect and swap. No drum puller, no springs, no guesswork.
First, Which Kind of Drum Brakes Do You Have?
This is the fork in the road, and it decides how much of the conversion is "bolt-on" versus "rewire." There are two common starting points:
Not sure which you have? Look at your coupler. A bulky coupler that slides and has a fluid reservoir is a surge (hydraulic) setup. A plain fixed coupler with a 7-way plug running to a brake controller in your truck means electric drums.
Part 1 — The Disc Brake Kit (Go Slip-Over)
For a conversion, a slip-over kit is almost always the right call. The rotor slides over your existing hub and is held by the wheel — so you don't have to pull hubs, replace bearings, or re-pack anything. That's what turns this into a weekend job.
Integral kits (rotor and hub cast as one piece) are the better choice if your bearings are worn out and you were going to replace the hubs anyway, or on a fresh marine build where you want everything new. If you're not sure your axle's specs, our disc brake kit chooser walks you through the four numbers — axle weight, lug count, stud diameter, and bolt pattern.
| Your axle | Slip-over kit (per axle) | Price |
|---|---|---|
| 3.5K · 5-lug · 1/2" | 3.5K Slip-Over Kit | $300 |
| 6K · 6-lug · 1/2" | 6K Slip-Over Kit | $583 |
| 7K · 8-lug · 1/2" | 7K Slip-Over Kit | $461 |
| 8K · 8-lug · 9/16" | 8K Slip-Over Kit (incl. idler hubs) | $479 |
| 3K · 5-lug · Tandem | 3K Tandem Slip-Over Kit | $1,399 |
| Marine 7K · Triple | Marine 7K Triple Conversion Kit | $2,499 |
Remember to buy one kit per axle. A tandem trailer needs two single-axle kits (or one tandem kit); a triple needs three. Don't see your exact spec? Call us — we stock rotors, calipers, and brackets individually and will bundle the right combination.
Part 2 — The 1,600 PSI Actuator (Don't Skip This)
This is the single most-missed part of a drum-to-disc conversion. Here's the physics: drum brakes engage at low pressure, so drum actuators are built to produce roughly 1,000 PSI. Disc calipers need about 1,600 PSI to clamp the pads onto the rotor with full force.
Reuse your old 1,000 PSI drum actuator on a fresh set of disc brakes and the result is brakes that feel soft, stop long, or barely grab at all — even though every part is new and installed correctly. The rotors aren't the problem. The pressure is.
| Actuator pressure | Designed for | Use on disc? |
|---|---|---|
| 1,000 PSI | Drum brakes | No |
| 1,200 PSI | Drum / light disc | Marginal |
| 1,600 PSI | Disc brakes | Yes — correct choice |
Electric drums → disc
Add an EOH actuator that outputs 1,600 PSI from your brake-controller signal.
Shop the 1,600 PSI EOH Actuator →Not sure which PSI?
Our full breakdown covers 1,000 vs 1,200 vs 1,600 PSI and how to match one to your brakes.
Read the PSI Actuator Guide →Parts 3 & 4 — Lines and Wiring
Brake Lines & Fittings
Calipers plumb differently than drums. Budget for new flexible hoses at each caliper, hard line where it runs along the axle, and the tees/fittings to connect them. Brake line tubing and fittings aren't included in the kits — they depend on your trailer's layout.
Wiring (EOH conversions only)
If you're coming from electric drums, the EOH actuator needs power, ground, a brake-controller signal, and a breakaway connection. Our conversion actuators ship as a plug-'n'-play kit with the 7-way plug, breakaway switch, battery kit, and junction box so the hookup is straightforward.
The Conversion, Step by Step
Here's the overall flow for a slip-over conversion. Always work off jack stands, never a jack alone, and consult your kit's included instructions for torque specs.
- Lift and secure the trailer on jack stands and remove the wheel.
- Remove the drum assembly — drum, shoes, springs, and backing plate. On surge setups, cap or disconnect the old hydraulic line.
- Mount the caliper bracket to the axle flange using the supplied hardware.
- Slip the rotor over the existing hub and reinstall the wheel loosely to hold it in place (slip-over) — or install the new integral hub/rotor and re-pack bearings (integral).
- Install the caliper and pads over the rotor and torque the caliper bolts to spec.
- Plumb the brake lines from the actuator back to each caliper.
- Mount the actuator (1,600 PSI EOH or surge) and, for EOH, wire power, ground, controller signal, and breakaway.
- Bleed the system starting at the caliper farthest from the actuator, until fluid runs clear with no air.
- Test at low speed in a safe area before towing loaded. Confirm even, firm braking on every wheel.
Time estimate: For a competent DIYer, plan on 1–2 hours per axle for slip-over, 2–3 hours per axle for integral (you're re-packing bearings), plus time to mount the actuator and bleed the system.
Five Mistakes to Avoid
1. Keeping the old actuator
A 1,000 PSI drum actuator can't clamp disc calipers. Upgrade to 1,600 PSI.
2. Wrong bolt pattern
Confirm axle weight, lug count, stud size, and bolt pattern before ordering.
3. Not bleeding fully
Air in the lines feels like a soft pedal. Bleed farthest caliper first, until clear.
4. Skipping the breakaway
The breakaway kit is required by law and applies brakes if the trailer detaches.
5. Wrong brake fluid
Use DOT 3 or DOT 4. Don't mix in DOT 5 (silicone) — it's not compatible.
Quick FAQ
Can I reuse my existing drum brake actuator?+
Almost never. Drum actuators produce about 1,000 PSI; disc calipers need roughly 1,600 PSI to engage fully. Reusing the old actuator is the #1 reason a fresh disc conversion feels weak. Plan on a 1,600 PSI actuator as part of the job.
Do I have to replace my hubs and bearings?+
Not with a slip-over kit — the rotor slides over your existing hub, so you keep your current hubs and bearings. Choose an integral kit only if your bearings are already worn out or you're building fresh (common on marine trailers).
How much does a full drum-to-disc conversion cost?+
Budget for the disc kit(s) — roughly $300–$600 per axle for single-axle slip-over kits — plus an actuator and brake-line hardware. A single-axle surge conversion is the least expensive; a tandem or triple EOH conversion is more because you're buying multiple kits plus the actuator system.
Is disc worth it for a boat trailer specifically?+
Yes — it's the single best upgrade for a saltwater trailer. Open rotors rinse and dry, while drums trap water and corrode from the inside out. Pair a slip-over or integral marine kit with stainless hardware and Dacromet-coated rotors for the longest life.
Do I need one kit for the whole trailer or one per axle?+
One kit per axle. A single-axle kit covers both wheels on one axle. A tandem trailer needs two (or one tandem kit); a triple needs three. Send us your axle count and specs and we'll confirm the exact quantity.
Planning a Conversion? Let's Spec It Together
Tell us your axle rating, lug count, stud size, bolt pattern, and whether you're on surge or electric drums. We'll build the exact parts list — kit, actuator, and hardware — so nothing gets missed. Free, every business day.
Talk to a HydraStar Techor call (812) 655-4544 · Mon–Fri, 8am–4:30pm ET