New Brake Lines/MC - Questions

[b2948kevin]

Ok, I have my disc brakes in and am ready to route the brake lines and get the MC in. Below you see the MC/Proportioning Valve combo that I got off of a newer chevy.

Questions:
1. One line is bigger than the other. What gives? Do I need to keep the same size line throughout, or can I just use a bushing to reduce/increase the size however I want to?

2. Do the front brakes always feed off of the front port of the MC (AKA the back port of the proportioning valve I have here, as they swap places if you look closely)? Any reason to switch these?

3. Someone mentioned that there's a tool for unlocking proportioning valves for bleeding. Am I going to need this? Is there an easy workaround?

4. This MC/Prop Valve combo will work for my truck (along with Captains adapter, which I already have), right?

[cb1965]

Small line, 3/16 is for the front . 1/4 line is for the rear

[Captainfab]

The front port on the master cylinder feeds the rear brakes, and the rear port the front brakes. Once they go thre the proportioning valve, it would be front to front and rear to rear. As cb1965 said you need 3/16" line for the front and 1/4" for the rear. That booster/master combo should work just fine. If you gravity bleed the new lines before the bleeding process, there shouldn't be a problem with the proportioning valve.

[b2948kevin]

Thanks for the responses. So, what if I want to reduce the rear line to 3/16"? can I just adapt it at the prop valve and go with it?

Also, I just measured the pushrod length through the firewall with the booster mounted up. It looks to be 3 3/4" too short. What is the best method to extend this?
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[cb1965]

I dont think i have seen 3/16 line for drums. I think it would be too much pressure for the wheel cylinders but dont know for sure.

[b2948kevin]

So, as I understand, the reason that you need to bleed air out of the brakes is that air compresses much easier than brake fluid, which is designed not to compress. Because of this, any pressure applied to the system at the MC should be equal to that applied to the brake cylinders at the other end of the system. This should be true whether the lines are 3/16" or 1", or reduced or increased to any diameter.

Am I incorrect here? If not, the next question is: why are there different port sizes on a MC? Just to keep them separated and not mixed up, perhaps?
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[bama63]

on my unmolested 63 the 1/4 inch brake line to the rear ran across the cross member then at the frame on the pass side connected to a reducer and 3/16 line all the way to the rear brake hose. so i do not see how a short length of 1/4 inch line made little if any difference in the braking so i ran all new 3/16 line and a corvette master and it stops great

[Jeff La]

By reducing the line size you will momentarily reduce the pressure going to the rear brakes and in essence would extend the time the rear brakes engaged. What you would be doing is installing an orrifice (3/16 tubing) and regulating the flow of brake fluid going to the rear of the truck. Would this really affect the braking, probably not, but I think you may wear out the fronts faster than design.

[Captainfab]

The factory ran 1/4" line from the proportioning valve all the way back to the hose that drops down to the differential, where it is split to 2- 3/16" lines going to the left and right wheel cylinders. I would say that was done to provide enough volume for 2 wheel cylinders. On the front of a split system, there 3/16" lines from each side directly to the proportioning valve.

As for extending your booster pushrod, there are a few ways to accomplish that. The easiest is to trim both the booster pushrod and your original pushrod to the correct length, and weld them together. Another way is to trim the booster pushrod, and thread it with the correct size threading die. Then you can extend it with a coupling nut and a piece of B7 all thread. Instead of threading the booster pushrod, you could alternatively couple the 2 pieces with a steel tube sleeve by cross drilling and installing roll pins thru each piece.

[b2948kevin]

Thanks a ton for the replies on this. I think this gives me enough information to move forward with. I am hoping to have this project done soon.

Also, captain, the booster bracket fits perfectly. Absolutely excellent adapter.

As always, I really appreciate the help here!

[Jeff La]

The bigger line is run to the rear to compensate for the longer run. More volume for a greater distance versus smaller line, shorter run for the front. It's designed to equalize the braking system. At least that's how it was explained to me.

[joesscamaro]

What year truck did you get your booster and master cylinder and proportioning valve from? I'm looking to do the same. Thanks and good luck with yours.

[b2948kevin]

You know, I really wish I could remember. It was an 80s fullsize pickup, that much I do remember.

I suspect that CaptainFab (and likely some others) could tell us everything we would ever want to know about this combo.

[joesscamaro]

That's what I was thinking, that it came out of an 80's something truck. Maybe someone will chime in and say which combo will bolt up. I'm going to my local pick n pull tomorrow and am putting together a list.
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[b2948kevin]

Yeah, I suspect so. Also, remember that you'll have to get an adapter plate (captainfab sells a really good one) and extend the pushrod on that brake booster by cutting it and rewelding your old one on there. In my case, I have to extend 3 3/4" to match it up. Not a big deal, but it's something to keep in mind.

Keep an eye out as I'll be doing this when I get back home and posting in my build thread. I'm currently on business travel.

[chevy_mike]

Regarding brake lines, how everyone is saying is the "normal" thing, 3/16" front, 1/4" rear. I figured this was standard.

Well, on my '65 Panel, it threw me for a loop. Factory single pot MC, 1/4" line to the first tee on the driverside frame. 1/4" line to both front drums BUT on the pass. side tee, the branch line that went to the rear is a 3/16" line all the way back. Flex line and axle lines are all 3/16".

Totally caught me off guard and I had never seen this done. My only guess is GM was trying to add a little more line pressure to the rear, since there is more weight in the back on a Panel truck than a standard pickup. One of those weird things you find...

[joesscamaro]

Originally Posted by b2948kevin

Yeah, I suspect so. Also, remember that you'll have to get an adapter plate (captainfab sells a really good one) and extend the pushrod on that brake booster by cutting it and rewelding your old one on there. In my case, I have to extend 3 3/4" to match it up. Not a big deal, but it's something to keep in mind.

Keep an eye out as I'll be doing this when I get back home and posting in my build thread. I'm currently on business travel.

I'll definitely be on the look out for your build thread. Captianfab is who I was going to go through to get my adapter plate as well. Still hoping someone will chime in on what year MC/Booster/Pro. valve will work. I did some searching and saw where someone said, from '81-91 will work. Would like to know for sure though.

[gimmy 64]

there is a looooong thread on doing this swap. captfab lists a bunch of master cilynders that work and boosters. go to the search thread and you will find it

[CRGRS 66]

being a "hydraulic guy" for a living, I would explain the 1/4" line for the rear as helping to make up for losses in flow and pressure through the longer trip to the rear brakes. They could not have engineered a diameter tube that would be too small to be effective, and would not have engineered anything bigger than was necessary. Definitely stick with 1/4" to the rear, trust the engineering that has worked for all these years.