[T2] Can disc master cylinder be used with drum brakes?

[david raistrick]

so the residual valves on all the bus MCs that I've seen (keep in mind it's
been a lot of years now since I last dug into this) are external.  so
they're easy enough to remove (or gut).

you definitely need residual valves on the drums - but any VW bus master
from any era is going to have that already because everything was always
rear drum.   even if you use a rabbit MC, it's also going to have rear
drums, so it'll have residual valves.   you'd be up into A4 golf land (99+)
to get a 4 wheel disc MC that wouldn't have resid valves...



for front discs - I don't really think a residual valve is a good thing at
any level.  there are no springs pushing things back, so you'd just be
using that residual pressure to hold the pads against the disc....


as for 1 2 or 4 piston calipers - for a bus, it really doesnt matter
(really).  (if you've got a 2liter in a split and you're heavy on the loud
pedal, it might matter....but not really).    1 piston calipers are always
floating calipers, though - so that's actually a real positive for daily
driver types.   what it means is that the caliper slides on a bracket and
-always- self centers (as long as the slides dont bind from lack of
maintenance and lube).    4, and usually 2 piston calipers, are fixed
calipers - you HAVE to shim and adjust them to center and square.   if
you're off a little bit, you'll get reduced brake force on the inside or
outside pad and uneven wear.

but our choices are limited (unless you're designing a kit yourself), so
it's kind of a "take what you can get" set of options.


I think the most important part of any disk brake selection is going to be
wear-item availability.  pads and disks and caliper piston seals.    So a
caliper that has fitment to a known vehicle (98 golf, etc) or from an known
aftermarket mfr (wildwood, etc) is really important - both from a long term
ownership standpoint and from a "can I adjust this with different pad
compounds if I dont like it".       Discs are harder - but a full size or
cut-down 944 disk, or a mustang 2 disk that has a machined seal surface are
things you -can- buy again and get any machine shop to make fit for you
later.   well, the full unmodifed 944 disc doesnt even need that. :)


I dont recall that EMPI had an option when I last looked - so not sure what
they're using.  Send me a link and I'll take a look when I get a chance.






On Thu, Jul 2, 2020 at 12:35 PM Warren McKean <warren.p.mckean at gmail.com>
wrote:

> David,
> Thank you for this info. I read your analysis of the 4 disc conversion
> kits plus other sites on the internet. There seems to be some agreement
> that either a 2 lb residual valve or no valve is recommended for discs and
> a 10 lb residual valve is recommended for drums. Since I will have front
> discs and rear drums I would need a master cylinder with no residual valves
> and install a 10 lb valve in the brake line to the rear drums. What do you
> think?
> Do you have an opinion of the advantages of getting a 1,2, or 4 piston
> caliper? And did you look at the EMPI kit?
> Warren
>
> On Wed, Jul 1, 2020 at 12:56 PM david raistrick <keen at icantclick.org>
> wrote:
>
>> yes.
>>
>> but you can also use the drum master for the disks.   for aftermarket
>> disk conversions, this is usually the better choice anyway.  (I've written
>> a lot about this in the past....)
>>
>>
>>
>> On Wed, Jul 1, 2020 at 3:47 PM Warren McKean <warren.p.mckean at gmail.com>
>> wrote:
>>
>>> Gents:
>>> I have a 1970 Baywindow (bought new). I plan to upgrade to front disc
>>> brakes soon, but have an immediate need for a master brake cylinder. Can
>>> I
>>> use the brake cylinder that will be necessary for the front discs with my
>>> existing front/rear drums? Also, since most folks install discs in the
>>> front only, does the 1971 brake cylinder have a retention valve in the
>>> rear
>>> hydraulic line?
>>> Warren McKean
>>> 1970 Camper
>>> _______________________________________________
>>> type2 mailing list
>>> type2 at type2.com
>>> https://www.type2.com/lists/type2/listinfo
>>
>>