The Stovebolt Site Forums The Shop Area Driveline Huck brake dual MC upgrade

The dual master cylinder project on my restored ’36 Chevy pickup is nearing completion. A compact dual MC with a 1” bore and left side ports has been acquired. The wide mounting ears have been machined to a much narrower configuration to allow the new MC to mount in the space previously occupied by the original MC, and a mounting bracket has been made of 3/16” steel plate to use the same mounting casting as original. A hybrid pushrod has been made to fit the OEM clevis and the new dual MC. The project is now at the point of plumbing the dual MC to the original front and rear brake lines. Other than making some tight bends and making some double flare terminations to a couple of sticks of new ¼” tubing that should be pretty simple.

Some information on various forums suggests that a proportioning valve and/or residual pressure valves are required with use of a dual MC and other information contradicts that. Since this is a 1” bore MC replacing a 1” bore MC, mounted it the exact same location, it seems that nothing is changed hydraulically except the increased safety of hydraulically isolating the front and rear systems. Front/rear pressure bias is determined by the front and rear wheel cylinder bores and those remain factory original.

My question is do any of you have any actual experience (not “opinions”) in doing this on a Huck brake system that would indicate that a proportioning valve and/or residual pressure valves are required for this conversion?

Thanks!

Are the stock brake lines different in diameter in the front compared to the back on a '36? That was a way that Chevrolet compensated for front-to-back calibration in some years. They would use smaller diameter lines in the back compared to the front.
Carl

Carl,

All the hard lines are 1/4". I've had this PU since 1965 and restored it in the early 1970s. The original brake lines were quite rusted so I replaced them all but the size then was 1/4", as it is now.

I thought the way front/back brake pressure bias was set was by using different bore size wheel cylinders.

An original equipment type proportioning valve (not the mickey mouse aftermarket type) has an isolator piston built in which is essential for the dual master cylinder to operate properly. When there is a pressure loss to one end of the system the piston moves to block off fluid flow to that line and preserve flow to the lines that can still contain pressure. Without the isolator piston the whole concept of the dual-circuit master cylinder is worthless. The valve you need to use will have an idiot light switch. Whether or not you choose to connect the light is up to you, but the valve needs to be there, no matter if the system is Huck or Bendix.
Jerry

The subject of tandem master cylinder plumbing is very complicated as shown in this article by a Studebaker guy:

http://www.studebaker-info.org/Tech/brakes/bf50014.html

And he doesn't even mention residual pressure valves, a subject on which "experts" totally contradict each other.

That's why I'm wondering if anyone has put a tandem master cylinder into a '36-'50 Huck system on a Chevy or GMC.

OK- - - -do it, and report on your results. Jerry

Are you drum/drum?

Often the original MC has a built in residual pressure valve as part of the piston or outlet fitting. See if yours did, if so then a new MC with residual valves or separate ones in line would be a good idea. The idea is to keep a little pressure (usually 10psi for drums) in the line to prevent drain back to the MC and a soft pedal, but not so much pressure the shoe return springs can't overcome and retract them.

I'd also use the more modern factory GM brass proportioning valve as Jerry describes, one for drums and drums if that is what you have. I've only used them for disc and disc, a similar looking but different valve I think.

DADS50,

What a great video! Thank you for sharing. It really tugged at my heart strings because I also had the "help" of little ones in my project (I'm a grandfather now) and I also did my total restoration at home except for chrome plating, glass cutting and engine cylinder boring and bearing pouring.

And I found out that a motivated amateur like you or me can outdo the paid "professionals" and "experts". Thinking that a car show was just a gathering that people drove their cars to I drove my just finished '36 Chevy PU from Southern California to the 1976 VCCA National Meet in Colorado Springs. It was a distance of over 1300 miles across the desert Southwest and over 12,000+ foot high passes in the Rocky Mountains.

Only when I got to the show did I realize that I may have bitten off way more than I could chew. My competitors brought perfect looking shiny antiques in enclosed trailers towed by motor homes. And mine? Dirty from the long drive and bug splattered. All I could do was rinse it off at the coin car wash and dry it with a bath towel. But guess what? It won Class T-2 (1929-36 Commercial Vehicles) at that National show.

So, here's to guys like us who do the work ourselves and pass those skills on to our kids and grandkids.

And thanks again for that great video. I see you also used a tandem MC but upgraded to disc front brakes. Very cool!

Where in Nevada do you live? Reno?

Grigg,

Thanks for that note. The available space for a MC on a stock '36 PU is tiny. Fortunately I have a great local brake supplier who knew of a very compact tandem MC. By narrowing the mounting ears I was able to squeeze it in.

All that remains to do is plumbing the new MC to the lines that exit the "T" fitting on the right frame rail. That's why I'm researching what that plumbing should include. Information on that subject varies greatly and in some cases is totally contradictory. That's why I'd so much like to find someone who's actually done it.

And that "combination" valve you refer to may be applicable but it's possible that using the wrong item may be worse than not using one at all. Do you know of a specific drum-drum compatible item? Some of those are "hold off" valves that are for disc-drum applications only because they delay front brake application to keep the front wheels from sliding on ice. There's so much to know about this that it makes my head spin.

You are correct, some years they used different bore size cylinders, front to back to achieve front/back bias.

http://www.jalopyjournal.com/forum/...or-dual-m-c-with-drum-brakes-wtf.646567/

I haven't waded through the whole thing but there is some good discussion in there. Perhaps your answer is in there too.

Grigg

Good morning Grigg.

http://www.jalopyjournal.com/forum/threa...kes-wtf.646567/

That article covers both residual pressure valves and proportioning valves. I've been researching everything I can find on this subject for the past few days. What I've found is just like the article, a bunch of contradictory information. For example, some assert that even with a drum/drum tandem MC system a RPV is essential in each circuit. Others say no.

This research does indicate that the RPV is to prevent drain-back to the MC if the MC is below the wheel cylinders. On my stock '36 Chevy PU the MC is above the wheel cylinders.

The symptom of drain-back supposedly that it will be necessary to pump the pedal to stop rather than simply pressing down on the pedal. I'm going to proceed without the RPV in each circuit and see if I have that symptom. That should tell the story. I did that with my '32 Ford roadster street rod. I ran it without a proportioning valve and made a "panic" stop that locked the rear wheels. Adding an adjustable proportioning valve solved that problem.

I wish someone had already done the same conversion I'm doing on my '36 PU but that is not the case so far. Thank you everyone for all your input.

Sometimes (often actually) when I get stuck and can't figure out the solution I research and look at what the "factory" did in a similar situation. Find a vehicle that has much of what your trying to use and find what else they used in that system, how did the factory do it.
You still need some understanding and judgement of your own but can benefit a lot from the factory engineering.

What vehicle used your new MC, or what vehicle has a dual circuit MC about that size and 4 wheel drums? Any valves or distribution blocks on them?

Good morning Grigg.

The MC I have is from a 1970s era Jeep CJ and was originally firewall mounted. I'm using it under the floor in the original '36 Chevy location.

I totally agree with you about finding out what the original factory setup was. Unfortunately I'm an example of a an analog person in a digital age. Despite my sons and now my grandkids trying to teach me computer skills very, little sticks.

That's why I was hoping to find someone who's done a conversion like mine who's example I could follow.

Some MCs have residual valves behind the inverted flare seats but the one I have does not.

In theory, at least, a RPV is required where the MC fluid level is below the wheel cylinder fluid level. That is not the case on my '36 although parking on a steep hill could cause that problem.

When I got my 50 back from the family friend it had a dual M/C under the floor.

He had split the circuits ,but was still drum/drum


...pic...

No prop valve or residual valves

Unfortunately I dont have the info on the M/C


I got rid of that set up and went with front disc brakes with a prop valve in stock location.

not much help to you
Good luck

I am going to comment on the need for residual valves. I have read the official explanations from GM and Ford about the valve. They are used to keep a positive pressure in the system and keep the seals active. I have yet to hear of a factory setup that didn't have them.

These days, every time I check a new MC to see if they installed an RPV, there is nothing there...it's as though they have found yet another way to save a buck on rebuilding/manufacturing them= and this is on MC's that would have had at least one RPV from the factory.....and the replacement part # does not have them!

I have seen RPV's in some MC rebuild kits, though.

The "Corvette" style universal dual MC that people often use- I have had 3 of them, and none of them had an RPV in them.

To all who have followed this thread here's what the same question generated on Chevy Talk.

http://www.chevytalk.org/fusionbb/showtopic.php?fid/26/tid/341390/pid/2668609/post/last/#LAST

Hopefully this will be useful to others considering a tandem MC upgrade or doing any other brake work.

Project update:

I just rigged up some plumbing to apply back-pressure into the original '36 Chevy MC and discovered that it has a 15 psi residual pressure valve in it. That removes all doubt whether the tandem replacement MC needs RPVs.