Dual Circuit Master Cylinder Upgrade For Pre-'67 Busses

72-79-bus DCMC.jpg (39652 bytes)

    Making your bus safer is a good thing.  Driving today in the midst of speeding, cell phone jabbering, SUV drivers can be down right dangerous at times, especially in a bus equipped a stock braking system.  When you hit the brakes, you expect to slow or stop the bus; you need to have complete faith in the pedal.  Well, your stock brake system's master cylinder (MC) is a single circuit master cylinder (SCMC), meaning that ALL 4 wheels are provided pressure by a single piston.  This also means that if you spring a leak or blow a wheel cylinder seal or hose, you will loose all braking capability.  Let me repeat that: YOU WILL LOOSE ALL BRAKING CAPABILITY!!!!!  This is because fluid is "lazy" and will take the path of least resistance which is out the leak!!

    Our fine German friends at VW realized this short-coming and changed to a safer design in 1967 with the addition of a dual circuit master cylinder (DCMC) These were standard on ALL subsequent bus/Vanagon models.  The advantage to this type of MC is that if you have, for example, a failure at one of the REAR wheel cylinders, you will ONLY lose braking to the REAR wheels.  The front will be unaffected by the failure since the front and rear are now two completely separate systems.  THIS IS GOOD!!  This means you won't totally loose your brakes and you will still be able to stop and hopefully avoid a collision or worse. 

    Does this sound like a worthwhile upgrade to you??  If so, read on. . .


1 - DCMC from a '72 - '79 bus

1 - Reservoir from a 70s Volvo (P/N - 678091)

1 - 1/2" spacer (optional)

2 - Longer MC mount bolts (required if spacer is used)

1 - Shortened brake pedal actuator rod (required only if spacer is NOT used)

Approximate cost: $40 - $60 depending on where you buy parts.


    First, I removed the old SCMC from the bus.  Pretty painless really since there is just 2 - 13mm bolts and nuts which hold it to the frame bracket, 3 - 11mm brake lines and 2-wires on the brake switch.  Here is a shot of the stock SCMC in the bus.

old-MC-installed.jpg (51362 bytes)

    Then I readied the DCMC on the bench.  First I thoroughly cleaned and fluched the MC with clean brake fluid as well as the reservoir too.   Here's a couple shots of the reservoir.  Note that this unit was taken from a 70s Volvo and is a PERFECT fit.  Many folks will tell you to use a Rabbit reservoir (which will work), but it's wedge shaped from front to back since the MC in a Rabbit is tilted up (this angled top sits flat on a rabbit, but will sit angled if installed on a bus).  The last photo is the P/N embossed on the side of the reservoir.  These may still be available new from your Volvo Dealer, but I don't know.

volvo-reservior.jpg (48078 bytes)            reservior-fit.jpg (37409 bytes)            reservior-fit-2.jpg (47745 bytes)            volvo-reservior-2.jpg (31501 bytes)

    With everything clean, all it needs is the cap.  The stock bus cap will fit perfectly on the Volvo reservoir, I'm going to be upgrading this as well to the Volvo unit which incorporates a low fluid sensor.  It's a really simple unit which has a float hanging from it.  If the fluid ever gets low, the float will drop about 2mm - 3mm and cause power to flow through the electrical circuit which will go to a light on the dash.  The previous owner of my bus was so "kind" as to add a couple holes along the base of the dash, so this will fill one up nicely and be functional as well.

filter-cup-n-cap.jpg (50822 bytes)            filter-cup-install.jpg (43120 bytes)            filter-cup-install-2.jpg (32935 bytes)

    Now the DCMC is ready for installation.  Here is a couple shots of it and the old MC side-by-side.  Which one would you trust more??

DCMC-ready.jpg (32652 bytes)            MCs-side-by-side.jpg (39554 bytes)

    Now here is where you must make a decision.  You can either add a 1/2" spacer to the front-side of the MC, OR you can make a shorter actuator rod.  I would NOT recommend cutting your original rod since these are really hard to find in good condition and could be used by someone else.  If you decide to make a shortened rod, get a metric bolt and cut it approx. 1/2" - 3/4" shorter than stock.  You will also need to file the end round like the original rod as well.

    I opted to make a spacer and leave the original rod in place (which makes it possible to adjust the rod length without removing the MC).  I found a chunk of 1/2" aluminum plate and bored a 1.5" hole with a hole saw and a hand drill.  Yes, a WOOD hole saw.  I'm not proud of it, but hey, in about 5 minutes I had a nice hole and it was cheaper than buying a Bridgeport mill and tooling!! :-)  I also had to bevel one side so that it would sit flat against the mount flanges on the MC.  The other spacer is 1/4" thick and was ready if needed, but was NOT needed or used.

MC-spacers.jpg (51604 bytes)            beveling-spacer.jpg (45092 bytes)            spacers-fit.jpg (49238 bytes)

    Next, I removed the actuator rod and applied anti-seize to the threads prior to re installing it.  Once it was installed, I added a dab of grease to the end where it contacts the MC actuator piston to help reduce wear on the rod.

anti-seize-on-rod.jpg (47501 bytes)            rod-installed-greased.jpg (57241 bytes)

    Now all I had to do was to install the DCMC using the longer bolts and connect the lines.  I hooked the front brakes to the two ports on the rear end eliminating the extra brake switch.  

Note:  The brake switch bore IS, in fact, properly machined for a flare fitting and was NOT just an open hole like on the SCMC.  If your MC is NOT properly machined for a flare fitting, you MUST either use an adapter fitting, OR run a "T" fitting from the port on the other side. 

    Make certain that you are hooking up the left FRONT brake line in the correct port.  The single line going to the rear brakes is routed parallel and it's easy to get them mixed up.  

Note:  Ensure ALL lines are routed well clear of moving parts like the clutch cable, steering drag link, etc.

    Then I screwed in the 2-spade brake switch in the right front port and hooked up the wires to the brake switch.

    It was time to adjust the actuator rod so that I had approx. 1/32" free-play and tightened the jam nut.  Here's a couple shots of the newly transplanted DCMC in my '66 bus.

DCMC-installed-L.jpg (56563 bytes)            DCMC-installed-R.jpg (51355 bytes)

    Now all that is left to do is to add fluid, bleed the entire brake system, re-check the adjustment on the actuator rod and give it a test drive.  Here's a view of the top of the reservoir from the access hole; PERFECT!!  Just like stock!! 

access-to-reservior.jpg (35419 bytes)


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Revised: August 23, 2003 .