Dorman M39952 Brake Master Cylinder Instructions

Table of Contents

Dorman-M39952-Brake-Master-Cylinder-FIG-1

Step Bore Brake Master Cylinder Bench Bleeding

Secure the master cylinder in a vise by clamping the mounting flange, avoiding the bore area to prevent distortion.
If non-threaded protective caps are present on outlet ports, replace them with threaded solid bleeder plugs.
Tighten bleeder plugs securely.
Gently tap the casting (not the reservoir) with a plastic screwdriver handle or wooden dowel to encourage air bubbles to escape from the bore during the bleeding process.
Fill the reservoir with new brake fluid.
Slowly depress the master cylinder piston to expel air bubbles from the bore. Repeat this process until no more air bubbles are visible.
Top up the reservoir as needed to maintain proper fluid level.
Check for leaks and repair if any are found.
Verify proper brake operation before driving the vehicle.

Why is bench bleeding necessary for the Step Bore Brake Master Cylinder?
Bench bleeding is required to remove air from the cylinder bore before installation to prevent air from entering the rest of the brake system.
Can I reuse old brake fluid when bench bleeding the master cylinder?
No, it is essential to use only new brake fluid from a sealed container to ensure optimal performance and safety.
What should I do if I accidentally use DOT 5 rated brake fluid in an ABS equipped vehicle?
If DOT 5 rated fluid is mistakenly used, flush the entire brake system to remove all contaminated fluid and replace it with the correct type recommended for ABS equipped vehicles.

ATTENTION – Refer to the appropriate service manual for your vehicle to obtain specific service procedures for this part. If you do not have a service manual or lack the skill to install this part, it is recommended that you seek the services of a qualified technician. Pay special attention to all cautions and warnings included in the service manual! Read and follow all instructions carefully! ABS equipped cars may require special bleeding and installation procedures! Consult the service manual for specific procedures.
WARNING! This Brake Master Cylinder MUST be bench-bled before installation on the vehicle! This process will remove air from the cylinder bore. Failure to bench bleed the cylinder will result in air being introduced into the rest of the brake system. DO NOT OPERATE VEHICLEUNTIL NORMAL BRAKE OPERATION IS OBTAINED!
WARNING! Use only new brake fluid from a sealed container! Do not reuse old brake fluid! DO NOT USE DOT 5 rated brake fluid in ABS equipped vehicles! Foaming of the brake fluid will take place during ABS operation. Do not mix different DOT rated brake fluids! Flush the entire brake system to remove all contaminated fluid.

Secure the master cylinder in a vise by firmly clamping the mounting flange on the casting. DO NOT CLAMP THE BORE AREA OF THE CYLINDER! DISTORTION OF THE BORE CASTING WILL OCCUR!
If the outlet ports have non-threaded protective caps installed, replace with the provided threaded solid bleeder plugs. Tighten bleeder plugs by hand to seat. Note: The plugs must be tightened enough to keep air from entering the master cylinder through the ports, but over tightening will result in stripping the plastic threads and rendering them useless.
Fill the reservoir with the appropriate brake fluid.
Using a blunt instrument or wooden dowel to avoid piston damage, slowly depress the piston into the bore using no more than 1 inch stroke. Do not bottom out piston.
Slowly release the piston to rest position. Wait 15 seconds to avoid fluid aeration.
Repeat steps 4 and 5 until air bubbles cease to appear in both reservoir chambers and the piston cannot be depressed any more than 1/8 inch.
The unit is finished with the bench bleeding process. Ensure reservoir lid is secure and release the master cylinder from the vise. Proceed to installation instructions.
TECH TIP: Gently tapping the casting (not the reservoir) with a plastic screwdriver handle or a wooden dowel will encourage the air bubbles to escape from the bore during the bleeding process.BRAKE MASTER CYLINDER

IMPORTANT: When bleeding step bore design master cylinders, you must wait 15 seconds after releasing piston to avoid fluid aeration.

Leave the bleeder plugs in the ports and install the master cylinder on the vehicle.
Tighten brake line nuts to the specification found in the service manual. Do not over-tighten! Damage to threads may occur!
After installing the master cylinder, flush the entire brake system with fresh fluid from a sealed container.
Refer to the service manual for specific bleeding/flushing procedures, especially when dealing with split-diagonal brake systems and ABS-equipped vehicles.
Check for leaks and repair if found. Maintain proper fluid level in reservoir.
Check for proper brake operation before driving the vehicle.

TECH TIPS

When bleeding the brake system, ensure the master cylinder is level. The rear of the vehicle may have to be raised if the master cylinder is mounted on an angle.
Use caution when handling brake fluid. Prevent contact with skin, face, eyes, and painted surfaces.
DO NOT , under any circumstances, introduce any petroleum based fluids (gas, engine oil, power steering fluid, transmission fluid, etc.), to the brake system! Failure (swelling) of the all rubber components will occur!
Brake fluid absorbs moisture. Never reuse old brake fluid! Always use fresh brake fluid from a sealed container.
If bench bleeding the master cylinder was successful, and brake pedal becomes softer after installing to the vehicle, verify the entire brake system. The issue is not the master cylinder.