Disc Brake upgrade - Do you need a new master cylinder ?

[Chopchop]

Hi

According to CB Performance, you do not need a new master cylinder when upgrading to front disc brakes.
They say the four-wheel drum brake master cylinder will work.

Obviously, they are experts and I am not but this just doesn't make sense to me.

Is it true ?

Has anyone installed their front disc brake kit and used a 4-wheel drum brake master cylinder ?

Dave

[KTPhil]

Most disc brake systems for VWs, including those form the factory, want ZERO residual pressure in the wheel lines. Drum systems, on the other hand, maintain a little pressure even with your foot off the pedal. There are "residual pressure valves" installed on the master for this purpose. They should be removed if you put on disc brakes.

If the disc brake kit says otherwise, call them and ask about this issue, then go with what they tell you.

[Chopchop]

CB Performance told me on the phone that the drum brake master would work fine. I can't see how that can happen.
In your opinion, do you think I should buy a disc brake master cylinder for this kit or can I use the drum brake master ??

[mtbmikey]

I used the drum brake master when I installed my discs. It had real bad drag after a few minutes of driving and braking. I opened up the master, removed the residual valve and it works great now. I still have drums in the back and adjust them till there is drag then back off a quarter turn each adjuster.

[Chopchop]

How do you remove the residual from the master cylinder ?

[Bruce]

If your car has ever had it's MC replaced in it's life, you don't need to worry about any residual pressure valves. All the replacements sold in the last 2-3 decades didn't have them.
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[Chopchop]

I don't know if the master has ever been replaced.

I went to Advance Auto and Auto Zone today and this is what I've found out :
The 4 wheel drum brake '68 Beetle master is the same part number as the disc/drum brake master from a '74 Karmann Ghia.
This master cyl. has a 3/4" bore diameter (approx. 19mm)

One of the foreign car parts places I stopped at told me that there were two diffent master cylinders used on '60s era VW's. There was a 17mm bore and a 19mm bore.

I don't know which master I have (17mm or 19mm) but I suppose from what you guys have told me and from what I've found out today, as long as I don't have a residual pressure check valve in my master, the existing master cylinder will work with the disc brake conversion.

Does that sound right to you guys ?
If I have a residual check valve and I end up having to remove it, can I do it while the master is in the car or do I have to pull it out ?

Is removing the residual check valve a tough job ?

[Bruce]

[Chopchop]

It's been stored since 1979. I'm redoing it and amazingly enough, with the exception of one rear wheel cylinder, the entire braking system is in excellent condition.

[KTPhil]

I have seen them mounted externally like in this drawing (#20). Just unscrew them and re-attach the lines (then bleed of course).

[Chopchop]

Excellent !
Thank you very much for that diagram.
I will check the master and if it has the check valves on in, I will remove the one that is in line for the front.

Thanks again,
Dave

[Chopchop]

A quick update :
That excellent brake system I wrote about ? - The master cylinder crapped out on me yesterday.
I'm STILL WAITING for my parts from CB Performance. Week 2 now.
I will update this post as I go through my disc brake update with my new master cylinder.

Dave