Dual Master Cylinder Conversion for 1955-67 (1950-55 also?) Bus

Dual Master Cylinder Conversion for 1955-67 (1950-55 also?) Bus

by Eric & Barb

List of needed parts:

  1. 1971 or later disc brake master cylinder
  2. Short metal brake line
  3. Two longer master cylinder mounting bolts
  4. VW Rabbit or 1967 only bus brake fluid reservoir
  5. Optional spacer
  6. Optional bolt (in place of cutting original push rod)

Total cost is about $50.00 and a couple of hours under your bus, resulting in a much safer braking system.

If you have a 1967 bus and want to use a master cylinder that's much less than 1/5 the price or you have a pre-1967 bus with single circuit brakes, here is a conversion for you and your bus's safety! Remember that on the single circuit brake system if a single slave (wheel) cylinder, flexible brake line, metal brake line (yes, metal brake lines can rust from the inside out!), or even the master cylinder leaks half the system dry or bursts from the up to 1200PSI in the brake system: you loose all hydraulic brake control. "So what," you say, "I have an emergency brake!" Have you tried to stop with only the emergency brake? Especially on a downhill or from high speed in a short distance? Now try imagining having one of the rear slave (wheel) cylinders burst and soak the brake linings on that side with slippery brake fluid, resulting in not only no hydraulic brakes but only half of the wimpy E-brake working properly! With dual circuit you end up with at least half of your brakes still working in such an emergency instead of almost none.

The best conversion for the 1955-67 (and this would probably fit just the same in the barndoors, pre-March 1955 buses) is the 1971 and later dual circuit brake cylinder for disc brakes. For the pre-dual circuit buses you need a brass T-fitting and a short metal brake line to it (about 100-150mm, or 4-6" long, which can be scavanged from the front disc brakes of a 1971 and later bus).

Rig the two front brake metal lines into a brass T-fitting with a short metal line going to the outlet on the underside of the master cylinder. This brass T-fitting can be scavanged from any VW from where the single metal brake line running to the rear splits into the two lines going to the rear wheels. In other words, plumb the two front metal brake lines up just like a 1967 bus front brake lines. Be careful not to mix up the rear metal brake line with the left front metal brake line. They both run right next to each other for several feet and are easy to mix up, resulting in a bus that pulls to the right when braking.

Then plumb the short metal brake line for the front brakes to curve around and under to the port that is under and at a 45 degree angle out of the bottom of the 1971 & later master cylinder. This outlet in this late master cylinder in its stock application is for one of the brake light senders, but if the sender is used in pre-1968 bus you can not install the belly panel which keeps the dirt, dust and rocks from the master cylinder, cables and linkages for controling the bus. Make sure to bend the metal brake lines in curves, not angles that would pinch off the flow.

You need a reservoir from a Rabbit, or better yet, a 1967 only bus reservoir. Check your local wrecking yards for either of these. These reservoirs are a snap fit into the seals in the master cylinder. To remove, grip them firmly with both hands and pull as straight up as possible. When you install either of these resevoirs put the fill cap end closest to the piston end of the MC so you can refill the resevoir through the stock hole in the bus front floor. Go ahead and use two brake senders in this new master cylinder and use the sender for the rear brakes as your primary. If this sender fails on a trip you can just switch the brake wires to the front brake sender, and replace the rear one as soon as you get back to civilization.

The two mounting bolts for the master cylinder will have to be 12.5-19mm, 1/2-3/4" longer than stock. You will have to shorten with a hacksaw and file the push rod and the bracket it threads into so the lock nut does not touch the master cylinder piston, or like one fellow says, use a shorter bolt and cut off the head instead of wasting the original push rod. Either way, make sure to file the shorter push rod end so it has the same rounded end as the push rod did before, and the shortened bracket flat so the lock nut sits flatly on it.

While all this will work this way it is bugger to adjust the push rod. Make sure to adjust the push rod for about .75mm, or 1/32" of play between the push rod end and the master cylinder piston. Check often to make sure this play still exists over the first 62KM, or 100 miles. If the play disappears expect brakes that drag and lock due to not releasing brake fluid pressure. You end up having to unbolt the master cylinder to adjust the push rod due to lack of room. If you have time now or later, build a 6-7mm, or 1/4" spacer (must be more than just some washers around the mounting bolts!) to go between the master cylinder and its mounting bracket on the bus to make the push rod far easier to adjust. Whichever you do, do NOT cut the front of the master cylinder piston at all. When all is installed, make sure that the metal brake lines are a minimum of 25.4mm, or 1" away from and moving linkages and cables or you run the possibility of having a brake line worn through. Have a friend operate your pedals, shifter, and emergency brake while you lie underneath to make sure that nothing gets too close!

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