What to use?

[69protour]

What to use? I've been researching which master cylinder to use and I'm at a dead end. I've got the Wilwood 12.3", 4 piston system front and rear. The camaro was a manual brake setup to begin with. So I called Wilwood and they said the new manual master cylinder that is a 1" bore and is already proportioned to 69%-31% will work fine on the car, with a prop. valve on the back just to fine tune it. Now the only thing is that the master cylinder is $200.00 alone. Now I found a master cylinder from Touring Classics with a 1" bore from a 67 to 76 corvette for $85.00. Could I use the same proportioning block off the car or go with one off an early 4 wheel disc corvette, and also add a prop. valve on the rear also to fine tune them? Thanks guys. Trond

[68BNUT]

Im gonna have to say no on the original manual drum brake proportioning block. You'll need a new one of those. Inline tube sells one for disc/disc application thats 91 dollars I beleive. Then you could use the Touring classics 1" master cylinder. You could even get a bigger bore master cylinder. I have 1 1/8" bore with willwood fronts and 4th gen rears and that can get a little tough to push sometimes. Check hotrodheaven.com for master cylinders. Good luck!

[fuzzyonion]

Use the drum distribution block and an adjustable rear prop valve.
I think (but not positive) the early Corvette distribution blocks are also just that.... a distribution block and warning switch, so are basically the same as what's on your Camaro.

Inline tube's or others disc/disc valve is spec'd for a specific brake system on a specific car. Is your car that car? No. Do they provide specs? No. Would anyone know what specs you need? Doubtfully.

If they say the disc/disc valve works on all disc/disc, I dont see how it is possible. Some systems/cars need more reduction and some need less. If it cuts too much, you'll have braking power restricted that could otherwise be used. If it cuts too little, you might have rear lock up.

The Corvette 1" master is very close to the 69/31 which your vendor says will work. It has a deep hole for a long pushrod, so make sure your booster rod or manual brake rod is the proper length.

[69protour]

I talked to the guys at Wilwood and they said don't go any larger than 1" bore, or it will get hard to push. Your right 68bnut, about getting hard to push the pedal. He said the bigger the bore the more fluid it pushes and the lower the line pressure is. I think getting a disc/disc block and prop. valve would be the way to go. Thanks guys! Trond

[fuzzyonion]

Yes, pressure and volume are proportionately inverse. Too large bore master=hard pedal, too small=soft pedal.

The aftermarket disc/disc combination valves are modeled after the 1979 Trans Am, which had a 58/42 system and ran at pressures approximately 50-800psi, had a light rear and heavy front. If you use a combination valve, do not double up by also adding an adjustable valve.

[69protour]

Your right fuzzy. But what do you mean by don't double up on valves? I was thinking if the block was of that proportion I might have to fine tune the proportion, being it's a completly different car? Oh by the way, I called O'Reileys Auto parts store and they have a 68 corvette master cylinder, 20.00 rebuilt or 64.00 new! Now I just have to locate a prop. block.

[fuzzyonion]

If you run a combination valve, it may already have a proportioning valve in it, so if you add an adjustable, and you then would have 2.

First thing to think about, and you may already know all this but I will say it anyway, is that a proportioning valve can never add pressure, only decrease it. If it is full open, that's all the pressure you can get, all the master cylinder will give.

Then think about how it works. It has a 'split' or 'knee' point. This is a pressure(psi) either predetermined or adjustable, under which the rear brakes get full pressure, and over which the pressure is reduced at a specified rate (slope).

The adjustable ones can adjust the knee point, but not the slope. Stock style is non adjustable.

Okay, the reason not to double up...
One reason is you now have 2 knee points. This might make it difficult to set correctly.
The main reason is you would get double the lag when you let off. With a pressure sensing proportioning valve, when you let off, the outlet pressure does not reduce at the same rate as the inlet pressure. It has to wait until the inlet pressure is equal to or less than the outlet pressure. With 2 valves you would effectively double the lag, and brake modulation may be difficult (the lag is the same reason to never run one for the front brakes).
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On the stock style T/A combo valve, it cuts enough pressure to keep rear brakes from locking up on that system. It is a comparitively low pressure system with a high front end weight and large rear brakes. I dont have the specs, but this tells me it very likely cuts quite a bit of pressure. I dont know what size your rears are compared to the fronts, but on the T/A, the rears are 42% of the hydraulic system at the wheel end.
According to some reliable sources, if you go below 35% on a Camaro you might not even need a proportioning valve, depending on some other factors. If you are above 42% rear, the T/A style valve might not cut enough for you... though if you are running a higher pressure system it still may be adequate (because it begins reduction at a comparative low pressure).


There are a lot of things you can do which will work, even work well, but in the end there are much fewer options for what will work 'best', and we all want best, right?