The VW Jetta is a versatile car and the most popular Volkswagen model sold in the U.S. When replacing the brake master cylinder in a VW Jetta, you should bench bleed the new one before installing it to ensure there isn't any air in the system that can get into the brake lines. The process only takes about 10 minutes; these directions apply to a VW Jetta from any year.
Remove your old master cylinder before you bench bleed and install the new one. The brake master cylinder of a VW Jetta is generally located up against the driver's side firewall, behind the tube coming from the air filter. You may need to remove the top of the air filter as well as the tube to remove and install the master cylinder.
Set your new VW Jetta master cylinder in a bench vise; if you don't have a bench vise, you can use a clamp-on vise and an old table. Clamp the cylinder firmly into place, making sure that it's level. Open up the bench bleed kit and have it close by.
Put the old reservoir into the new master cylinder, if you're reusing it. Be sure you dry off the reservoir completely, since brake fluid absorbs water and can damage your brake system, and then install it into the top of the new master cylinder. If a reservoir came with your new VW Jetta master cylinder, you can skip this step.
Find the two fittings that came with your bleeder kit. Thread them onto the outlets located on the side of the cylinder. Take the two lengths of hose that came with your kit and insert them into the fittings. Bend the hoses up, so they're aimed into the fluid reservoir.
Cut the hoses, if necessary, so they stick point-down into the reservoir and extend about halfway into it. Use a clip to secure the tubes to the side of the reservoir and keep them in place. You don't want them to come loose and allow air into the system or spray brake fluid around.
Fill the VW Jetta's reservoir with fresh brake fluid. Pour enough into the reservoir to fill it just shy of the maximum fill line. The plastic hoses will extend down into the fluid, creating a closed hydraulic system.
Pump the piston on the brake master cylinder to move the fluid through the unit and into the hoses. Use a Phillips screwdriver to do this by putting it into the cylinder and pushing it firmly against the piston to start pumping.
Watch for air bubbles coming out of the hoses and into the fluid in the reservoir. Keep pumping until all the air is out of the cylinder and you don't see any more bubbles. Consider buying clear hoses if the ones that come with your kit are black, so you can see the air bubbles better.
Leave the two hoses in the reservoir and slowly remove the cylinder from the vise. You can now install the primed master cylinder into your VW Jetta.
