203-07040 MC808269 Mitsubishi Vacuum Booster FUSO Truck Parts

203-07040 MC808269 Mitsubishi Vacuum Booster for Fuso Truck

1. Quick Notice

203-07040 / MC808269 vacuum booster for Mitsubishi Fuso medium and heavy-duty trucks. Tandem diaphragm design for hydraulic brake systems. Direct bolt-on replacement for original vacuum booster unit. No master cylinder or mounting bracket included. Verify booster mounting pattern and pushrod length before ordering. Professional installation required.

2. Description

The MC808269 vacuum booster is a pneumatically assisted braking component designed for Mitsubishi Fuso trucks equipped with hydraulic brake systems. This booster is commonly installed on Fuso Fighter, Super Great, and The Great models using 6D16, 6D15, 4D33, or 4D34 engines. The booster amplifies the force applied by the driver's foot to the brake master cylinder, reducing pedal effort significantly during service braking.

The booster features a tandem diaphragm configuration. Two separate rubber diaphragms operate inside a single sealed housing. Each diaphragm has its own vacuum chamber and atmospheric chamber. The tandem design provides approximately twice the assist force of a single diaphragm booster without increasing the housing diameter. This configuration is essential for heavy trucks where pedal force requirements would otherwise exceed acceptable driver effort.

The housing is constructed from cold-rolled steel with a corrosion-resistant e-coat finish. The exterior finish resists road salt, moisture, and chemical exposure common in commercial vehicle underbody environments. The housing is seam welded and leak tested at the factory to maintain vacuum integrity for the service life of the unit.

Internally, the MC808269 contains a control valve system that regulates the amount of atmospheric pressure entering the variable pressure chambers. When the brake pedal is at rest, both chambers on both diaphragms are under vacuum. When the pedal is pressed, the control valve opens progressively, allowing air to enter the atmospheric side of each diaphragm. The pressure differential between the vacuum side and the atmospheric side generates the assist force. The output pushrod transfers this force directly to the master cylinder piston.

The booster includes a built-in air filter at the atmospheric inlet. This filter prevents dust and debris from entering the control valve and diaphragm chambers. Contaminated air causes control valve sticking and premature diaphragm wear. The filter is not serviceable. Complete booster replacement is required when the filter becomes restricted.

The mounting flange uses four studs arranged in a square pattern. These studs pass through the firewall or mounting bracket. Nuts secure the booster from inside the cab or from the engine bay side depending on chassis configuration. The input pushrod connects to the brake pedal linkage via a clevis and pin. The output pushrod length varies by chassis. Verify output pushrod length before ordering.

This replacement booster is suitable for vehicles experiencing hard brake pedal feel, hissing noise from the booster area with engine running, brakes that release slowly after pedal release, or engine stalling when brake pedal is pressed. A failed booster does not eliminate braking function entirely but increases required pedal effort to near manual brake levels.

3. Parameter Table

4. Custom Content

Installation notes for MC808269 vacuum booster. Disconnect battery negative cable before starting. Remove master cylinder from old booster. Support master cylinder to avoid stressing brake lines. Do not let master cylinder hang by brake pipes. Remove four mounting nuts from inside cab or from booster flange. Pull old booster forward and off firewall studs. Clean mounting surface completely. No gasket is used between booster and firewall. Inspect one-way vacuum check valve. Replace check valve if cracked or if valve fails to hold vacuum. Transfer check valve and rubber grommet to new booster. Install new booster onto studs. Tighten mounting nuts to 18 to 22 Nm. Do not overtighten. Nut overtightening warps booster housing. Adjust output pushrod before reinstalling master cylinder. Pushrod must just contact master cylinder piston at rest. Gap of 0.5 mm to 1 mm is acceptable. Excessive gap increases pedal travel. No gap causes brake drag. Reinstall master cylinder onto new booster. Tighten master cylinder nuts to 15 to 18 Nm. Bleed entire brake system after booster replacement. Air in master cylinder affects booster performance. Test pedal feel with engine off and with engine running. Running engine should lower pedal height by 15 to 25 mm.

5. FAQ (Q&A Format)

Q1: Which Fuso trucks use the MC808269 vacuum booster?

A1: This booster fits Fuso Fighter, Super Great, and The Great models with hydraulic brakes. It is not compatible with air brake chassis. Verify your brake type before ordering.

Q2: How do I know if my vacuum booster has failed?

A2: Common failure symptoms include rock hard brake pedal with engine running, hissing sound from booster area, engine idle dropping when pedal is pressed, and brakes dragging due to control valve leak.

Q3: Is the MC808269 a single or tandem diaphragm booster?

A3: This is a tandem diaphragm booster with two separate rubber diaphragms. Tandem design provides higher assist force for heavier vehicles.

Q4: Does this booster include the master cylinder?

A4: No. The MC808269 is the vacuum booster only. The master cylinder must be transferred from the original unit or ordered separately.

Q5: What is the function of the vacuum check valve?

A5: The check valve holds vacuum in the booster when engine vacuum drops. It also prevents brake booster from leaking vacuum back into the intake manifold. Replace the check valve with the booster.

Q6: Can I install this booster on a left-hand drive Fuso?

A6: Yes, provided the original booster part number matches MC808269. The booster itself is not handed. The pedal linkage adapts to left or right drive configurations externally.

Q7: Why does my brake pedal feel normal with engine off but hard with engine running?

A7: A normal pedal with engine off confirms hydraulic system integrity. Hard pedal with engine running indicates complete vacuum booster failure. No assist is being generated.

Q8: How much vacuum does this booster require to operate correctly?

A8: The MC808269 requires a minimum of 18 inches of mercury engine vacuum at idle. Diesel engines with worn rings or valve seals may produce insufficient vacuum.

Q9: Can a leaking vacuum booster cause engine performance problems?

A9: Yes. A torn diaphragm allows unmetered air into the intake manifold through the vacuum hose. This lean air mixture causes rough idle, stalling, and black smoke on diesel engines.

Q10: What is the correct output pushrod adjustment procedure?

A10: With master cylinder removed, turn pushrod until it just contacts the master cylinder piston at rest. Back off one half turn. Brake drag indicates pushrod is too long. Excessive pedal travel indicates pushrod is too short.