New Master Cylinder

[ngneer]

On a 70 std. beetle, I have a dual circuit master cylinder that is leaking fluid and running into the tunnel. Question is, can the same newly replaced master cylinder be used when a disc brake upgrade is planned coming in late this winter or spring 2008? If I replace now can it be used with disc brakes? Thanks

[FJCamper]

You'll need a Ghia master cylinder made for disk front, drum rear, if you're keeping the drums.

If it's disks all the way around, you need a master cylinder specified for all-disk, or residual valve to keep some pressure on the pads.

Disk brakes need a slight amount of residual pressure to keep the pad in close contact with the rotor.

There are people who use a "standard" (drum-drum) dual circuit master cylinder on disk systems and get by with it, but always have some brake problems and think they're normal. For street, you might get by with it. For high performance, no.

[Marc]

The original-equipment `67 and `68/early`69 drumbrake master cylinders had discrete screw-in residual pressure check valves, but the "universal replacement" style used since late`69 (113 611 015BD) does not use them - it has restriction drillings in the outlet ports instead, allowing it to be used for both drum/drum Beetles and disk/drum 'Ghias and Euro-spec Beetles. There is no such thing available as a "'Ghia" M/C.
Disk brakes should not normally need residual pressure valves - in fact, they can cause drag and premature pad wear. The purpose of RPV's is to keep a slight pressure in the wheel cylinders of drum brakes, which aids in sealing - in my experience wheel cylinders tended to last longer with the old-style RPV-equipped M/Cs than they do with the "restriction-drilling" type, but there's really no difference in performance.
If you're experiencing piston withdrawal on disk brakes, it's most likely caused by rotor run-out. RPVs may mask the symptom but they don't solve the real problem.

[FJCamper]

Dear Marc,

You're right about there no longer being a Ghia-only master cylinder. I used that as a description because so many suppliers still designate m/c's like that. But, there is some clarification needed about residual pressure valves and how they should be applied.

Both drum and disk brakes need some residual pressure but for different reasons, and the reasons determine how much pressure is necessary. VW created a good compromise in the universal dual circuit m/c's with built in restriction valving.

Drum brakes have strong return springs to pull them away from the drum's swept area after contact, and barring wear, a fixed distance to travel to and from the drum. Shoes don't "retreat" at random distances to and from the drum.

On a drum brake, a very small amount of residual pressure in the lines does no more than keep the slave cylinders expanded a bit to seal better, as you pointed out, if nothing else is wrong.

But on a disk system, brake pads can retreat at random distances if the pad is pushed back (Wildwood calls this piston "knock-back") for any reason. Axle bearing movements, and brake spindle flex are the two big causes of this. We personally just suffered through this on our four-wheel disk brake '65 Ghia in the November 2000-mile Carrerra Panamericana race.

http://www.wilwood.com/Centers/Informat ... ooting.asp

Even a couple of milimeters of "knock-back" causes excessive pedal travel on the first pump, to take up the slack.

This can and does happen on race cars with understanding drivers and mechanics, but can't be tolerated on street cars. No manfacturer wants to be known for "bad brakes."

All disk systems provide enough residual pressure to keep the pad in light contact with the rotor, and we have to accept the associated wear in order to have "right now" brakes. Most people never even know it's happening.

Very few people install residual pressure valves on drum systems because they don't have to. Lots of people have to install residual pressure valves on disk conversions.

Many resellers, California Import Products among them, don't explain the need or purpose of the residual pressure valves they sell very well, and give ambigious info, particulary the part about application to drum brakes.

[ngneer]

Ok guys,not sure if I understand, the part # given, 113 611 015BD,, would this be what I need for now until the total rebuild, to be able to use it now, and with the rebuild. I may not have clarified myself, this will be front disc and drum on back. I understand the type 3 will be a bolt on for the type 1, if so with the part # I'll have another question. Thank you both for the input.

[FJCamper]

Dear ngneer,

Yes, the universal master cylinder should work fine on a standard disk-drum system.

We've left the question hanging about how and why a residual pressure valve is used on a drum system. Lots of problems get created in making a disk or drum brake race car, and a residual pressure valve can fix some of them.

For instance, the master cylinder should not be lower than the brakes themselves, to prevent brake fluid drainback. And racers sometimes mix master cylinders, slave cylinders, and other parts not originally meant to work together.

Even though a drum brake system has fixed shoe travel, a bleed-down and reduction of hydraulic pressure in the lines means an initially soft pedal feel.

A residual pressure valve keeps up the pressure longer. The average drum brake residual pressure valve retains about 10lbs. of pressure to overcome the brake return springs, but not press the shoes into the drums.

Disk or drum, the residual pressure in either system does bleed down over time, which means it's a good idea to gently press your brake pedal every half hour or so on a long interstate drive.

I didn't mean to write a book about this, but I have not seen much info on the reasons for and the application of residual pressure valves, and your question led into it.