Disc/Disc Master Cylinder Recommendations

[Tony71c10]

Question, when swapping over to rear disc brakes, does one need to replace the master cylinder or can I get by just changing the proportioning valve. Truck is 71. If I need to repace the master cylinder which one would be a direct fit? Thanks in advance

[jradford]

I think the only difference is the removal of the proportioning valve. Disc/disc shouldn't need it. The disc/disc may use a larger piston diameter but don't think so. I would look at some of the aftermarket conversion kits and see if they go to a larger master cylinder. Just a thought.

[davepl]

I used the parts out of 1969 Corvette. The master and combo valve are widely reproduced and available as service parts even.

[Mike C]

It definitely needs a disc/disc master, and like Dave said, C3 Corvette is ideal. A WS6 gen 2 Trans Am with 4 wheel discs (79-81) would work as well.

[Tony71c10]

Appreciate the info, corvette master cylinder with or without a combo valve. Would did be a direct swap, no need to make adjustment to rod on booster etc?

[RichardJ]

The bore of the Dual master cylinders is the same in the rear (primary section) as it is in the front (secondary section).
Pressure from the rear port is the same as the pressure from the front port. Always, unless the MC is bad.

If the MC, the booster and mechanical linkage is correct for the front calipers now, then it doesn't matter if the rear brakes are drum or disk.
The purpose of the proportion valve is to proportionally reduce the pressure to the rear drum cylinders. The pressure needed for the front calipers is too great for the rear drums and must be proportionally reduced to prevent rear brakes from locking up.

Adding discs to the rear, does not require as much pressure reduction to the rear as with the drums, but because of the light weight in the rear of a truck, it may need some reduction.
Many trucks use load sensing proportion valves.

If the MC is good for what you have now, then don't screw that up. Determine after the rear disc install, what pressure reduction to the rear, may or may not be needed.

[lolife99]

I now always try to use an aftermarket ALUMINUM master cylinder.
The cast iron M/C's will always rust, no matter what you paint or coat them with.
Then install a Wilwood or other aftermarket "adjustable" proportioning valve in the rear brake line.
I would do this whether it was disc/drum or disc/disc.

[davepl]

Originally Posted by RichardJ

If the MC, the booster and mechanical linkage is correct for the front calipers now, then it doesn't matter if the rear brakes are drum or disk.

I always wondered why it did - if the front calipers are the same, what's the change? But yet Chevy used a different master when the added rear discs to the JL8 Camaros, as far as I know.

[Mike C]

GM used the Corvette master cylinder on the JL8 cars. The calipers weren't exactly the same as the vette calipers, but functionally the same.

I have built a reproduction HD type brake setup for the back of my 68 Camaro convertible but haven't mounted and bled all the parts yet. I got repro JL8 brackets for the front and repro 'vette calipers all around. Because of the staggered rear shocks you need two right side Corvette rears and not a complete set of 4.

Drums have residual pressure valves in the master in some installations and that wreaks havoc on a disc conversion. If I was doing a 4 wheel disc swap on my own truck I would use a Corvette master and a Corvette combination valve or as I mentioned above, the late 70's early 80's 4 wheel disc 2nd gen F bodies.

[Mts67C10K20]

I am just finishing up installing 4w disks on my 67 k20
And am using a master cylinder from a 80 vette with a pv4 valve
Just make sure your using a booster with a short push rod (almost flush with booster face)
Not sure what a 71 came with
Here is link I found helpful
Fyi 14 bolt disk parts bolt up to an Eaton also
Good wrenching

http://www.fourwheeler.com/how-to/su...o-disc-brakes/

Apologies for the pic rotation