**Safety critical.** If your early Bronco still has a single-reservoir master cylinder, replace it. A single-circuit system means one leak anywhere — a blown line, a failed wheel cylinder, a bad seal — leaves you with no brakes at all. A dual-circuit master splits the system so a failure in one half still leaves you stopping power in the other. This is the single most important brake upgrade you can make.
Early Broncos from 1966 to mid-1967 left the factory with a single-reservoir master cylinder. Federal regulation required dual-circuit systems starting in 1967, so later trucks got them, but many early trucks have been through enough hands that you cannot assume what is installed. Look at the master cylinder: one reservoir chamber means single-circuit, two chambers means dual.
A single-circuit system has one hydraulic loop feeding all four corners. Lose pressure anywhere — a rock-torn line on the trail, a corroded fitting, a worn seal — and the pedal goes to the floor with nothing behind it. There is no backup. On a 50-year-old truck with original lines, that is a genuine risk, not a hypothetical.
A dual-circuit master cylinder has two separate hydraulic systems, typically split front and rear. A failure in one circuit still leaves the other working. You lose braking on two wheels, not four. The truck stops longer and pulls, but it stops. That difference is the line between an inconvenience and a crash.
The clean way to do this is a combined conversion:
1. **Dual master cylinder** sized for your brakes. If you run front discs, use a disc/drum or disc/disc master with the correct bore. Bore size affects pedal feel and pressure — match it to whether you have a power booster.
2. **Brake booster**, if you are converting to discs or want lighter pedal effort. A 7-inch dual-diaphragm booster fits the early Bronco engine bay with common brackets. Manual (unboosted) setups work but require firmer pedal pressure.
3. **Proportioning or combination valve.** A disc/drum system needs proportioning to keep the rear drums from locking before the front discs bite. An adjustable proportioning valve lets you tune the front-to-rear balance.
4. **New lines from the master.** Reusing crusty 50-year-old hard lines on a fresh master is a missed opportunity. Replace at least the lines coming off the master cylinder.
Bench-bleed the master cylinder before installing it. A master with air trapped inside will never give a firm pedal no matter how long you bleed at the wheels. Mount it in a vise, attach short bleed tubes back into the reservoirs, and stroke the piston until no bubbles appear.
After installation, bleed the full system in the correct sequence — farthest wheel from the master first — and confirm a firm, high pedal before the truck moves under its own power. A spongy pedal means air remains; do not drive until it is solid.
Get the bore size and proportioning right. Too large a bore gives a hard, grabby pedal; too small gives a long, mushy one. If you are unsure, match the master cylinder bore the kit vendor specifies for your exact brake configuration.
Budget $150 for a manual dual-master conversion reusing the booster, up to $500 for a full booster-and-master setup with new lines. There is no cheaper insurance on this truck. If money is tight, do the master cylinder alone first — the dual circuit is the part that saves you.
| Part | Vendor | Est. price |
|---|---|---|
| Dual reservoir master cylinder (disc/drum or disc/disc) | Various | ~$90 |
| Brake booster (7-inch dual diaphragm) | Various | ~$180 |
| Proportioning valve / combination valve | Wilwood / Various | ~$60 |
| Brake line and fittings | Various | ~$50 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.