Adding disc brakes to a drum car
 

I was going to install a power disc brake conversion on my ‘67 LeMans manual drum brake car but now I’m thinking of sticking with manual brakes vs power. Is it as easy as keeping my stock non power master & adding front rotors/calipers? Will I need to add a proportioning valve & keep the brake splitter block on the frame rail?

no master cylinder is different
yes
yes
yes

So there’s a different master cylinder for disc vs drums even without power?

Yes, the master cylinder is different. Unless you want an incredibly stiff pedal, the master cylinder for non-power disc car is a smaller bore.

If your engine makes decent vacuum, and you want power brakes, now would be the time to make that change. You're going to end up changing almost everything (including that hard to get proportioning valve on the frame) regardless.

Youll get some differing opinions here. There was just a big thread in the street section where several people said they used discs without swapping the factory distribution block and everything was fine.

I currently have a disc/drum dual master in a 4 wheel drum car that came with a single pot. Everything works fine. No proportioning valve.

I think what I would say is that if you are doing work anyway, bleeding brakes etc, a disc/drum master and either a combo valve for disc/drum setup or an adjustable proportioning valve to your rear line is pretty cheap. So why fight it.

On a drum/drum car is the "valve" mounted to the frame actually a portioning valve or is it just a splitter with a sliding piston to activate the brake warning light and sperate the front and rear systems in case of a failure? I've never been really clear on it's purpose.

end up changing almost everything (including that hard to get proportioning valve on the frame) regardless.

is the same drum and disc ,,,,, 71? it changes to incorporate the stay off valve

68 GTO 4 speed had a manual disc master cylinder,,,,,, and 4 piston calipers.... my convert had this factory

if I were you I would buy everything for a 69 ,,,,,

That’s the point I’m trying to make, I don’t want power brakes, I want to keep manual brakes but add front disc.

The block on the frame in 67 is a distribution block. Back in about '85 I added a 69 disc brake set-up with power to my 67 Tempest and all worked well.

Both from personal experience and sluething in threads like this, Ive come to belive that Disc/Drum is the only setup where you might need a true combo or proportioning valve. And even then I think its a maybe. A while ago for some project or another (I think it was my 64 galaxie) I was talking to wilwood or someone about what combo valve to use if I were going to still be drum brakes all around. They recommended their disc/disc valve.

FWIW the manual master in my Firebird is the "corvette" aftermarket variety that Right Stuff sells and it still has big reservoirs with a 1" bore.

Also, from experience with that car, I think 15/16 is the sweet spot for an Fbody weight car with manual brakes.

Because of course they do - they sell them!

Almost all GM combination valves sold are made for one car: 1971 Chevelle with front disc/rear brake. It of course does not matter whether the master is manual or power - same pressure and flow in and out.

The OEM's no doubt expended quite a bit of time dialing in bore sizes of the master/front/rear for optimum braking under most situations encountered by day to day driving, so in reality no combo or delay valve is needed.

I would agree with that. After much Quixotic windmill-poking trying to find an ideal OEM aluminum master for my TA I ended up with the 24mm Mopar minivan unit. This did require a bit of work to fit - too much actually.

The stock manual 1" bore works well enough; I suspect GM came to the same conclusion. Hence I used a 1974-80 GMC 1/2 manual unit: same hydraulics but far greater availability for my 71 Lemans.

https://forums.maxperformanceinc.com...1&d=1734608362

Absolutely. There were entire staffs of engineers and technicians dedicated to keeping it all going.

You have to meet FMVSS standards at both curb and GVW, in addition to any customer "pleasability" requirements, so the development and validation testing is nearly insurmountable.

K

I was just making the statement that a 4 wheel drum and 4 wheel disc setup are going to behave similarly. If all 4 wheels have the same configuration of brake, IMO you can pretty safely just be using the the factory jurassic era distribution blocks that came in most 4 wheel drum cars with lines just routed to the master cylinder as necessary. If a brake company is saying you can use the disc/disc combo valve for a drum/drum car. I feel like you can use the factory drum/drum block in a disc/disc car.

Im a little less certain when it comes to disc/drum setups, because its never been clear to me if the blocks that came in disc/drum cars from the factory are dumb hunks of metal, or if they have built in proportioning in them. Talking about GM engineering, I'm not sure what they engineered into the blocks in those cars. And maybe its not all the same, maybe some have proportioning built in and some don't.

Im not arguing, I agree with you that I would feel comfortable relying on the master to do what we need it to in most cases. Im just saying I don't feel comfortable stating you 100% for certain wont need some sort of proportioning device to the rear brakes when you switch from drum/drum to disc/drum. People should for sure listen to you over me, but I have done this a few times now and in different ways, and honestly, the brakes don't see to care very much how you do it. As long as they are getting hydraulic pressure from a pedal pressing a fluid filled cylinder, they do their job.

Disc/drum has proportioning (and metering). The constraints shown below are meter (ie hold off) / split point / slope.

And each one is different.

K

https://forums.maxperformanceinc.com...1&d=1734646111

Hasn't done me any good.

I have them all the way back to 1969 and I still can't get the brakes on my Chevelle like I want them. 30 years I've been messing with 'em.

K

https://forums.maxperformanceinc.com...1&d=1734646185

One thing I noticed long ago on my old Camaro with JL8 ( rear discs) is that cars with rear disc brakes generally Slow down about 20 -30% Faster than disc/drum. This holds true with the Scarebird brakes on second gens, and was true on Ws6 cars., which is almost identical. Its Still used today on most cars. When I say "slow down" < I mean Slow down, NOT STOP. Purpose of 4 wheel disc is to NOT lock up the brakes. It has got me out of a few bad situations with bad drivers and , well we have too many Turkeys ( literal ones) here.
I use mine with Dot 5 fluid too ( Gasps !!) yeah Dot 5.... no water and no ruining paint.
I ran into some big issue with GM truck engineering with a weight/fluid Limiter on the rear of my 1 ton truck back in 82. I was shocked how many different weight and F/R ratios they had calculated for their Combo valves, and limiters. Its amazing that it has come down to basically 1 Combo valve that's marketed for all.

Here's the brake chart for 1982, for full size truck:

https://forums.maxperformanceinc.com...1&d=1735662167

https://forums.maxperformanceinc.com...1&d=1735662194

https://forums.maxperformanceinc.com...1&d=1735662207

The brake RPOs are driven by 1/2, 3/4 and one ton models, in two and four wheel drive, with various wheelbases and GVW ratings, and manual vs vacuum boost vs hydraulic boost.

And here are the characteristics that are controlled:

https://forums.maxperformanceinc.com...1&d=1735662179

I would posit the main driver for all the variation is/was FMVSS testing. We had to pass stopping distance tests, at curb and GVW, or we could not sell a truck.

Some were so marginal that we would ask for a specific technician to run the test matrix (ie "I want Wayne to drive this one"). We would shotgun it with "development" tests until we got a pass, and would then label that run as "validation".

I'm not surprised the aftermarket tries to sell one part to cover all but that anyone in the industry would sincerely think that would work.

To me it's like selling one carburetor (and associated jetting and settings) and expecting it to cover all engine combinations.

K