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Discussion Starter · #1 ·
I currently have Front Disk / Rear Drum brakes on my 1970 Nova. I'm in the process of converting the rear brakes to disc. I know that a different proportioning valve is required but I'm wondering if the Master Cylinder needs to be changed as well. Any thoughts will be appreciated.
 

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There are 3 styles of GM dual reservoir cast iron master cylinders.

- 2 small reservoirs originally for drum/drum applications
- 1 big reservoir/1 small reservoir originally for disc/drum applications
- 2 big reservoirs originally for disc/disc applications

Thing is, you don't really need the big reservoirs for discs. What the big reservoirs do is this. The fluid level drops as the disc pads wear. With the big reservoirs, you (theoretically) won't need to top off the fluid level through the whole life of a set of pads. With the small reservoirs, you will need to top off the reservoir as the pads wear - maybe once. No big deal, just check the level a couple times a year. You should be doing that anyway.

So you can run a small reservoir with discs no problem except for that.

There are no other differences in the GM master cylinders that would make them special for discs.
 

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Your master cylinder made for disc/drum brakes will have a residual pressure check valve in the drum brake side. This valve holds 10psi of pressure in the lines to the drum brakes since the return springs retract the shoes fully. The small residual pressure helps maintain good pedal feel. If you use that master with discs, the discs will always drag and wear due to the residual 10psi of pressure (and no return springs). You should get a disc/disc master cylinder. I put a Wilwood master on my 63 with disc/drums and had to add an inline 10psi check valve per Wilwood's instructions.
 

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Discussion Starter · #5 ·
Thanks Jim! I’ve read that a GM Master Cylinder from a 67 - 69 Corvette (427 disc brake) is a good choice. I assume those year ‘Vettes had disc / disc brakes and it should bolt right in. Thoughts?
 

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Your master cylinder made for disc/drum brakes will have a residual pressure check valve in the drum brake side.
Maybe, maybe not. Actually not likely. An original master cylinder for drum/drum had them in both ports, but I've found that most rebuilds these days don't have them. Also a new aftermarket repro master cylinder won't have them either.

You can check to see if they are there. Put the smooth end of a small drill bit through the hole in the brass cone seat in the outlet port. Probe around and see if you feel rubber. If you do, that's the residual valve, it's a "duck bill" valve under the cone seat. To remove, run a 6-32 tap in the hole in the cone seat, then put a 6-32 x 1" long (or so) screw in the tapped hole. You can now put the claw end of a claw hammer under the screw head and pull the brass seat out. Remove and discard the rubber residual valve and the spring under it. Clean up any burrs on the cone seat and reinstall it.

A system that has a disc/drum master cylinder (big/little reservoirs) and a disc/drum combination valve has a residual valve in the outlet of the prop valve, none in the master cylinder. That's the primary difference between a disc/drum and a disc/disc combination valve. You can disable that residual valve too.
 

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JimStout, I am in the same boat with my 70 Nova right now. I too have front discs and drums and in the rear. My Nova also has the standard staggered shock setup in the rear. So, I am curious about what you are doing there. Are you buying a kit that works with staggered shocks or are you changing your shock config back there too?

Thanks.
 

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MC are cheap, get a new one. For manual brakes I use a 70 Vette MC and for Power brakes I use a 80 Vette MC. I believe they are different bore size. Works well on all of the swaps I have done. Bought a brand new one at NAPA last week for $50
 
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Discussion Starter · #9 ·
JimStout, I am in the same boat with my 70 Nova right now. I too have front discs and drums and in the rear. My Nova also has the standard staggered shock setup in the rear. So, I am curious about what you are doing there. Are you buying a kit that works with staggered shocks or are you changing your shock config back there too?

Thanks.
I'm looking at a kit from Right Stuff Detailing. They have rear disc brake kits for staggard and non-staggard shocks.
 

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disc brakes

"MC are cheap, get a new one. For manual brakes I use a 70 Vette MC and for Power brakes I use a 80 Vette MC. I believe they are different bore size. Works well on all of the swaps I have done."


When you convert to 4 wheel disc, do you still need a proportioning valve? If so which one. 63 Nova. Thanks!
 

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1963 Nova coupe, Phoenix Arizona
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"MC are cheap, get a new one. For manual brakes I use a 70 Vette MC and for Power brakes I use a 80 Vette MC. I believe they are different bore size. Works well on all of the swaps I have done."


When you convert to 4 wheel disc, do you still need a proportioning valve? If so which one. 63 Nova. Thanks!
I just used an adjustable inline one on the rear line only like below. Put it under the car so you don’t see it.

https://www.jegs.com/i/JEGS/555/630...OoT9CZoL5ZQ8-ANOMLx1_ohLozP1c2WhoCOyAQAvD_BwE
 
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