[Scarebird]

We changed a 68 FB with manual drum to manual disc a few years back. Out of curiosity we left the manual drum and distribution block in. Car stopped perfectly.

Note manual masters have smaller reservoirs and MUST be monitored for fluid drop with use.

[RocktimusPryme]

Originally Posted by Scarebird

We changed a 68 FB with manual drum to manual disc a few years back. Out of curiosity we left the manual drum and distribution block in. Car stopped perfectly.

Note manual masters have smaller reservoirs and MUST be monitored for fluid drop with use.

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.

[Scarebird]

Originally Posted by RocktimusPryme

...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...

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.

Originally Posted by RocktimusPryme

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

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.

[Keith Seymore]

Originally Posted by Scarebird

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

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

[RocktimusPryme]

Originally Posted by Scarebird

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.



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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.

[Keith Seymore]

Originally Posted by RocktimusPryme

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.

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

And each one is different.

K

[Scarebird]

Originally Posted by RocktimusPryme

...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...

Please let me clarify. What I recommended to our customers was bleed it well and try it out as-is: if it worked to their satisfaction - excellent, and this was overwhelmingly the case. If not, usually an adjustable delay valve in the rear line would trim it to where they needed to be. Ideally with our nose heavy cars you want the fronts to lock up a bit before the fronts to prevent the rears locking and spinning out. GM did this by different combo valve holdoff spring rates.

Other manufacturers, like Toyota and VW used actual valves with levers and linkage so when the rear rose up it cut off pressure to the rear brakes. I cannot comment on how well that worked.