A brake pedal that drops towards the floor is the one CJ340 fault you should never drive around. The system is simple — a master cylinder, steel lines, a flexible hose at each corner and 11-inch drums all round — but on a thirty-year-old jeep with no servo and, often, a single hydraulic circuit, one tired seal can take away braking at every wheel at once.
This guide covers the hydraulic side: reading pedal symptoms, diagnosing and replacing the master cylinder, choosing fluid, and bleeding the system properly. For shoes, drums and wheel cylinders, see our drum brake overhaul guide.
Read the pedal before you pick up a spanner
The pedal tells you which part of the system is failing. Learn the four patterns and you can usually point at the culprit before anything is dismantled.
A long pedal that firms up when you pump it is almost always shoe adjustment. CJ340 drums do not self-adjust, so as linings wear the shoes sit further from the drum and the pedal has to travel further before they touch. Adjust all four corners before touching the hydraulics — bleeding a system with badly adjusted shoes will never give a good pedal.
A spongy pedal that feels soft from the first millimetre means air in the system, or old fluid that has absorbed water and boils when the brakes get hot. A pedal that holds at first and then sinks slowly while you keep steady pressure at a junction means the master cylinder is bypassing internally: fluid slips past a worn piston seal instead of being pushed down the lines. That one is urgent.
A pedal that suddenly goes to the floor, with a fluid trail or a wet backing plate, means an external leak — a burst hose, a split wheel cylinder seal or a corroded steel line.
- Long pedal, firm when pumped — adjust the shoes
- Spongy from the start — air or water-laden fluid; bleed or flush
- Sinks slowly under steady pressure — master cylinder bypassing
- Sudden drop with a leak — hose, wheel cylinder or steel line
- Hard pedal, poor stopping — contaminated or glazed linings, not hydraulics
Single circuit, dual circuit and the remote reservoir
CJ340s left the factory with more than one master cylinder arrangement, and Mahindra parts catalogues list both a standard assembly and a version with a remote reservoir. Before ordering, check what is fitted: count the outlets on the master cylinder and see whether the reservoir sits on the cylinder body or is mounted separately and connected by a hose.
On a single-circuit system, one failure removes braking from all four wheels. That is why many owners who use their jeep regularly on public roads treat a dual-circuit conversion as a priority, and why inspectors in the UK, Germany and France look hard at the condition of every hose and line on a single-circuit car.
There is no vacuum servo on most CJ340s. High pedal effort is normal for the design. Soft or sinking is not.
Diagnosing the master cylinder
Start with the simple checks. Look at the fluid level and colour: dark brown fluid is old and wet. Look behind the master cylinder where the pushrod enters, and on the bulkhead or chassis below it. Fluid there means the rear seal has failed and fluid is escaping.
To confirm internal bypass, clamp the flexible hoses one at a time with proper hose clamps (never pliers) and press the pedal. If the pedal still sinks slowly with all hoses clamped, the problem is inside the master cylinder. If it firms up when a particular hose is clamped, look at that corner.
Check pushrod free play. There must be a small gap between the pushrod and the piston when the pedal is released. With no free play, the piston cannot return fully, the compensating port stays covered, and the brakes gradually lock on as they heat up — a fault that often gets blamed on seized wheel cylinders.
Rebuild or replace
Seal kits are cheap, and if the bore is smooth and bright a rebuild works. If the bore shows pitting or scoring — common on jeeps that have stood with wet fluid — honing removes the seal's working surface and the repair does not last. On a brake master cylinder, replacement is the honest answer whenever there is doubt.
Bench-bleed a new master cylinder before fitting it: mount it in a vice, fill the reservoir, and stroke the piston with the outlets looped back into the reservoir until no bubbles appear. It saves a great deal of time at the wheels.
When ordering, quote your chassis number (CJ3B-4 DK…) and say whether your jeep has the integral or remote reservoir. Nonu Ventures lists both the standard master cylinder and the remote reservoir version.
Which brake fluid
Use the glycol-based fluid the system was designed for (DOT 3 or DOT 4) and do not mix it with silicone DOT 5, which is not compatible with glycol fluid and should never be added to a system that has been running on it.
Glycol fluid absorbs moisture from the air. Over two years, enough water gets in to lower the boiling point noticeably and to start corroding bores from the inside. On an old jeep that is driven infrequently, a full fluid change every two years is cheap insurance for the wheel cylinders and master cylinder.
How to bleed CJ340 brakes
Adjust all four brake shoes first. Then bleed each corner in order, starting with the wheel furthest from the master cylinder and finishing with the nearest. On most layouts that means rear wheel on the passenger side, rear on the driver side, front on the passenger side, then front on the driver side — but check where your master cylinder sits, because left-hand-drive Bahias and right-hand-drive jeeps differ.
The two-person method works reliably: one person presses the pedal slowly and holds it down, the other opens the bleed nipple, lets fluid and air out into a clear tube and jar, and closes the nipple before the pedal is released. Repeat until the fluid in the tube runs clear with no bubbles. Keep topping up the reservoir — if it runs dry, air goes straight back in and you start again.
Old bleed nipples are often seized. Soak them in penetrating oil the day before and use a six-point ring spanner. A snapped nipple means removing the wheel cylinder, so be patient.
When the pedal is firm, road-test at low speed, re-check adjustment after the first few hundred kilometres as the shoes bed in, and look again for leaks at every joint.
- Adjust shoes at all four wheels
- Fill the reservoir with fresh DOT 3/4 fluid
- Bleed furthest wheel first, nearest last
- Never let the reservoir run dry
- Road-test gently and re-check for leaks
Hoses and steel lines
Flexible hoses perish from the inside. A hose can look fine outside while its inner lining collapses and acts as a one-way valve, holding pressure on one brake and causing a pull or a dragging drum. If the jeep has stood for years, replace all the flexible hoses — front and rear — as a set.
Steel lines rust where they run along the chassis and clip to it. Scrape back any bubbled coating: pitting on a brake line is a fail at every inspection and a real risk. Copper-nickel line is easy to bend and does not corrode, which makes it a popular replacement on restorations.
Frequently asked questions
Why does my CJ340 brake pedal go to the floor?
If it sinks slowly under steady pressure, the master cylinder is bypassing internally. If it drops suddenly, look for an external leak at a hose, wheel cylinder or steel line. If it is long but firms up when pumped, the shoes need adjusting.
What order do I bleed the brakes in?
Adjust the shoes first, then bleed from the wheel furthest from the master cylinder to the nearest. Check where your master cylinder is mounted, as LHD and RHD layouts differ.
Which brake fluid does a CJ340 use?
Glycol-based DOT 3 or DOT 4. Never mix with silicone DOT 5. Change it every two years on a jeep that is used infrequently.
Can I rebuild the master cylinder?
Only if the bore is smooth and unpitted. If there is any corrosion, replace the unit. Quote your chassis number and reservoir type when ordering.
Keep yours on the road
The part this guide covers — Brake master cylinder assembly — ships worldwide, OEM or reconditioned. Quote your chassis number (CJ3B-4 DK…) when enquiring so fitment can be checked.