Can it be done? I have all the parts for a disc brake swap but I won't be getting to it for awhile, but I really want to ditch the only single chamber M/C on my 66 Cuda. Eventually I would love to upgrade to the new aluminium master cylinder so if I can swap that on right now I would be a happy camper. Does anyone know of any problems that might occur if I use the newer MC with four wheel ten inch drum brakes? Proportioning valve issues? Chamber size issues? Pressure diferential problems?

Thanks!

Reed

Every source I've read says you can not use a disc brake master cylinder on a drum setup...and vice-versa.

You can but it just will make your pedal feel a bit wierd. Drum brake MC's have a residual pressure valve on each line meant to be used with drum brakes. It keeps a smidgen of pressure int he line to overcome the springs on the shoes. If you use a disc/drum MC on a drum/drum car, the pedal will probably travel a bit further untill the front shoes engage. It shouldn't be much though.

Reed, I recomend you get a rebuilt iron dual chamber drum/drum MC for $20 from your local kragens then worry about an aluminum upgrade later when you do discs.

Hmmm okay. I'll just go with a drum/drum unit. How about the prop valve? Anyone know if those are still available new or are they a boneyard item?

Reed

For a car with all drums, there is no proportioning valve, its just a distribution block (and housing for brake warning switch). The fronts and backs get equal pressure in an all drum car.

Similar ones are availible new from mpbrakes, but very expensive, last I remember checking it was $100 or so. I suggest junkyard. These usually don't go bad becasue there is still brake fluid in them to keep the rubber o-rings moist. And since the housing is made of brass corrosion is not likely. I believe you can have them sent out to get rebuilt but I can't remember the place I found on the net that did that.

When going from drum to disc, you have a couple of options regarding divvying up the pressures between the front and rear. You can remove your distribution block and replace it with a stock junkyard (or $100 MPbrakes unit) disc/drum unit. These distribution blocks have the proportioning valves built into them because of varrying width passages inside them.

Or, your second option, is to keep your drum distribution block, and then tee in one of those adjustable wilwood (and other various brands) proportioning valves into the line going to the rear axle.

On my recent conversion to KH discs up front I used a stock proportioning valve because I didn't want to cut the rear metal line and do flaring to install an adjustable aftermarket unit. Parts stores don't even list these valves. Maybe a dealer item or NOS parts house if your lucky.

Using a disc brake master cylinder with drums is pretty straightforward - you will just need to add a residual pressure valve to the line going to the front brakes. I understand that leaving this out can cause the seals to start leaking.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
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Alright......... I think it's way past time all of us get updated on current brake technology. Some years ago brake cylinder rebuild kits came with new "expanders" for the cups, and now today they use a revised spring that works as the expander so the residual pressure valves are no longer needed.
I'm not aware that any master cylinder for my 75 or 76 Valiant's with Drum brakes has an residual pressure valve. I will have to look if the service manual even says anything about them. I know the older service manuals DO point out the residual valves.

This is a subject that needs to be look into and we all need to be updated on.

It's no big deal with drum brakes because you can adjust the shoes. Just make sure with discs you don't have one or else you might drag.

The master cylinder I have on my Satellite is a disk type (small front chamber, large rear) that I got from a local parts store. Thats what the computer called for, so thats what I used with no problems with my 4 wheel 10" drums.

Eric I think the rebuild houses don't care about resevoir size in terms of MC's I bet they just use whatever ones they have handy and install/remove the residual pressure valves as called for. I have a Autospecialty brand MC for the 4 piston KH setup that the computer called for, and has equal size resevoirs.

Craig has the right idea here. If those drums were EVER rebuilt, Reed, then they have those wheel cyl springs and you don't need the resid valves.

I would recommend using the Alum MC with an adjustable prop valve ($33 from Summit right now) in the rear line - with discs or drums.

Lou

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Home of Slant6-powered fun machines since 1988

The comments ot Dart270 and Craig agree with what I have been told, and it is nice to have some confirmation of it here on this board. My local buddy Randy has been driving a 74 Duster, 66 Dart, and 67 Pontiac for years, all with the residual valves removed. He claims he has had FAR fewer problems with shoes wearing out prematurely, AND gets slightly better mileage with no residual valve. To remove it, thread a sheet metal screw or appropriate sized tap into the outlet on the master cylinder and engage the sheet metal insert that traps the residual valve spring. Pull out the retainer, and let the spring and valve fall out, and reassemble everything. It's as simple as that. I haven't done this to my cars, but will next time I open the brake lines.

My guess is that the aluminum MCs never had residual valves, and probably rebuilt iron dual pots also do not have them. This is just a guess.

I have been running an untouched alum MC on my 4-wheel disc Dart and have noticed no excess pad wear or braking weirdness.

Lou

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Home of Slant6-powered fun machines since 1988