brake question

I have a question; it might be controversial but i still have it. I was reading i think on here and can't find the thread. If you have a modern master cylinder from a mid 80s pickup in an older car do you need to run a proportioning valve? If i recall, there were a few guys saying with 4-wheel disc setup and new master cylinder the prop valve renders its self-useless?? I did a 4-wheel disc setup on my demon and just wondering as I will be doing a line lock and if i can get rid of the prop valve I will.

True or false? and any comments or experience is welcome.

I know I ask a lot of questions i should know the answers to.

I have 4 wheel disc and don't have one.
now having said that, it is supposed to be a safety item as well.

A proportioning valve is needed for disc drum systems. It's not required for drum drum or disc disc. It would be beneficial in those cases for fine tuning though.

BTW, that 80s truck MC has 2 problems with disc disc systems. 1 it may have a residual pressure valve, no bueno for disc brakes. 2 the reservoir for the rear brakes is much smaller than typically found on disc brakes, if you keep an eye on fluid levels this isn't a problem.

I have four wheel disc brakes on my 70 E Body, I installed a disc brake master cylinder and I am still running the proportioning valves. No issues after 12 years. I think you can do without it though.

Just throwing a mix of parts together and the answer is "maybe". I always considered them a bandaid. Sometime a necessary one, but still a bandaid.

Figuring out pedal ratios, bore sizes of the MC and the calipers along with the number of pistons to get total area of the bores, tire sizes (big tire drag car vs all tires relatively the same size), getting it all right, and putting it all together and it should not be needed,

You won't know for sure what you need until you install and test it. There are a ton a variables involved that cannot be assumed or guessed at. pedal ratio, piston sizes, drum and disc sizes, vehicle weight, brake lining friction coefficient, tires sizes, their compounds and the list goes on.
You will likely need the residual pressure valves as previously mentioned

You can install an adjustable if your replumbing the rears or roll the dice and see if you need one

This is all true. Starting completely from scratch and designing and specifying every aspect means you can do it without any proportioning valve. Think OEM research and development.

But, we're limited to what's available and it's going to be less than ideal. I recommend installing an adjustable valve from the start. Due to the differences in operating pressures, a disc drum system needs one no matter how much math and research you do.

Originally a 4 wheel drum or disc drum?

If Originally a disc drum setup, you're leaving alot of rear braking power unused.

If drum drum, you're good.

And, yet, the factory often uses them in disc/disc installations. IMO, the only way to know is actual testing...slam on the brakes at speed and see what happens.

100%

All true.

However, I would drive it first and see what is needed, or not...and where.

Why not include the entire statement. I did say it's good to have for fine tuning.

quote] I did say it's good to have for fine tuning.[/quote]

And the tune will change between a full tank and an empty tank. Unfortunately there is no “automatic” prop valve.

Many drag racers put the valve in the front brake line. An average street car should have it in the rear line. Race cars - other than drag cars - will have two masters with the bias adjustable in the linkage so adjustment can be done on the fly as conditions dictate.

Yes there is.

My turbo minivan had one. Will it work on our stuff? Dunno.

i was just going to mention that as well, but you treed me............
all the minivans i have owned or junked over the years has had one of those things. also wondered if adding one would work in other applications.

Those have been used on lots of different vehicles over the years. Problem is they're typically designed for a specific vehicle and the odds that they will be close for our stuff are slim.

Plus, they're all old. Pretty much disappeared when ABS became common.

Make sure if you are running a disc/drum master cylinder on a disc/disc setup. There isn’t a residual valve hiding in the rear brake port. Ask me how long I ran with one hiding in my port……

Also used on various 80's/90's pickups with rear wheel only ABS .

There is also the disc / disc master on a disc drum setup that will wreak havoc. That can be cured with an inline residual valve to the rears