Correct placement of proportion valve on '69 C20 front disc swap

[Caddylackn]

My truck had a disc swap when I bought it, but looks fairly hacked together, with lots of splices and adapter fittings. The prop valve is located on the frame behind the radiator. It still has the stock '69 master and brake booster.

I want to put the proper '71 master in it designed for front disc brakes and move the prop valve by the master and route the brake lines correctly but I need to know how a '71 C20 master, prop valve, and brake lines and brake line sizes.

What is the proper way to route the brake lines? The prop valve should be mounted by the booster. Is the prop valve from my '69 correct? or do I need one from a '71, or does a '69 not come with one and I already have the correct one?

Which side of the prop valve is for the front brakes, which for the rear?

I found this image but its not clear enough for me.

[Accelo]

Man, you stepped into one of the highly variables.

Seems the line location changed a bunch, some inside the frame and some outside. Seems the line location also varied by build location. It also depends on what exactly the prior owners changed to install the disks. Typically the lines for a drum brake truck ran behind the cross-member and the disk brake lines ran in front of it. Where are the lines now? Some retro kits move the lines most don't. Confirm all the existing stuff is stock or close to it.
Some installs use a longer diameter vacuum booster. Just a ton of variables.

Frankly I would likely start with a new prop valve and the lines from the master to the prop valve. They have special fitting that adapt a small line to a larger port and are somewhat difficult to find. Then I would hand bend the lines that go from the prop valve to the brake lines. I suspect this will be the least amount of hassle. inlinetube.com may be your best bet. They sell on eBay also. Don't forget you need double flairs and to hook up the electrical line to the prop valve. It is a brown wire and was used on both drum and disk trucks. Maybe you will find the remnants of it still there.
I personally found this to be one of the most frustrating parts of rebuilding my truck with disks. I have been through three front brake lines to prove it and this was supposed to be a plug and play kit. LOL.
Best of luck.
Rick

[Caddylackn]

I have working brakes with a prop valve now, I don't want to move any brake lines on the frame, I just want to move the prop valve up to the master where its supposed to be and replace the master with one for disc brakes. Where they have the prop valve now required at least three splices in the line and access to the prop valve behind the radiator sucks. I have new 3/16", 1/4" copper nickel line, and a bunch of flare nuts in all sizes, and a double flare tool. Time to make this right with no union nut splices.

I found this prop valve on Summit and it came with the bracket, pre-bent lines to the master, and a diagram of how to plumb;

https://static.summitracing.com/glob...sum-760183.pdf

Looking at later year master cylinders (MC), it is unclear if a '71 MC for a C20 will fit on my '69 C20 brake booster. I know that the MC of a disc brake circuit does not have a pressure residual valve PRV, but a MC circuit for drum brakes does, so the stock '69 MC needs to go. You don't want to run a PRV on disc brakes because the residual pressure will make the calipers drag a little. What I really need to do is just remove the PRV on the MC from the disc brake circuit. I found on the forum how to do this.

Is the disc brake circuit on a '71 MC the one closest to the firewall? or to the front of the car?

[RichardJ]

>> I just want to move the prop valve up to the master where its supposed to be <<

The '69 didn't have a Proportioning Valve, so it isn't supposed to be anywhere. The '69 had a distribution block. Summit is doing a disservice to you and everyone by calling that a Proportioning Valve. That's a Combination Valve.

'73 and up mounted the Combo Valve under the radiator. '73 and up had disc brakes.

If you look at your Summit listing it says the application is UNIVERAL.

Would you rather have a disc brake setup designed for a '73 Chevy truck or disc brake setup designed for Fnord or a Studebaker?

[RichardJ]

>>Which side of the prop valve is for the front brakes, which for the rear?<<

The rear outlet of the Combo Valve is the Proportioning end. The front two outlets are the Metering Valve and they are for the front brakes.

>>Is the disc brake circuit on a '71 MC the one closest to the firewall? or to the front of the car?<<

Many, but not all disc/drum MC have a larger reservoir for the disc brake outlet, but the important thing is that the pressure from both MC outlets is exactly the same. Technically, either MC outlet can be used for the disc calipers. The size of the reservoir only becomes important when the disc pads wear down to nothing and the reserve capacity of the reservoir is exceeded and it starts pumping air.

Your summit listing shows the Metering valve outlets. You should be able to follow those lines to the front brakes.

[Caddylackn]

I went to the junkyard last weekend and I was lucky to find a '72 GMC with the master cylinder (MC) and proportional valve (PV) still attached. The MC and PV were rusted junk but I snagged the bracket to hold the PV where it is supposed to be located. I then grabbed the splitter block for the front brakes. With a photo, (see photo 3 below) I was able to confirm how to set this up to match a '72 front disc/rear drum set up.

I installed the PV onto the bracket where it is supposed to be (see photo 2) and installed the splitter block for the front brakes where the PV was and the front brake lines ended behind the radiator by the frame rail. I then ran a new 1/4" brake line from the PV to the front splitter. Then I ran a new 1/4" brake line from the PV all the way to the rear crossmember for the rear axle and connected to a new rear brake hose. I can't say enough good things about that copper nickel brake line. I was able to get rid of three splices in the old rear brake line. These 3 splices were done with standard water line compression fittings, see Photo 1 below, and not double flare brake line connections so this was a ticking time bomb. I also removed another splice on the front brakes with the new 1/4" line. I ordered a 3/16" inverted flare plug for the PV, but I pinched off a piece of 3/16" Cu-Ni. brake line and soldered the end for a temp plug so I could bleed the system. It seems to be holding good brake pressure with no leaks. I used a reverse brake bleeder kit and it worked really well at purging all the air out of the new lines. I have got a firm hard pedal now and will test drive once I get my plug into the PV.


RichardJ- I agree with everything above except for

Originally Posted by RichardJ

the important thing is that the pressure from both MC outlets is exactly the same. Technically, either MC outlet can be used for the disc calipers.

The MC has a pressure residual valve (PRV) of around 10 psi inserted into the rear brake port of the MC from the factory. The constant 10 psi is to preload the brake drum springs and take out excessive travel in the linkage so there is not excessive brake pedal travel in order to activate the drum brakes. Either circuit can be used in the MC if there is a PRV on the drum circuit and as long as there is no PRV on the disc brake circuit. A 10 psi PRV for drum brakes on the disc brake circuit will cause slight disc brake drag. It is easy to take out a PRV by inserting a screw of the appropriate size into the seat insert on the MC (where the brake line seats) with a big washer. Then tighten the screw using the washer to pull up the seat and out, it is a press fit. Then just remove the rubber valve and spring (PRV). Then reinstall the seat insert. Just don't mess up the seating area where the brake line seats.

My original MC had a PRV in each port since it was all drums. The rebuilt MC I just put in, actually has no PRVs in it when I checked. I have heard and now confirmed that these PRVs are often not re-installed in the rebuilt MCs from offshore suppliers. I got this one from NAPA. You can check your MC for a PRV by installing a tiny screwdriver into the port and see if you can feel the rubber valve and spring by pushing on it. I can try to install the PRV from my old MC, or I am first going to try to run it without a PRV on the rear circuit and see if it works.

I am running a Dana 70U rear end with the larger chevy C30 wheel cylinder brake mod, so I should have plenty of rear brakes and may need a PV for the rear if I get early rear brake lock up.

[gmc684x4]

67 72 brake boosters are pretty much all the same length from the fire wall

Where they differ is inside 1/2 and 3/4 ton will have different force to apply to the brakes

C series k series same brake booster

Do your self a favour buy the full line kit from inline tube or another place like that obviously the previous owner just butchered original bake line would you trust your families life on hacked 50 plus year old brake lines ?

master cylinders will be different none power boost will have a deeper push rod hole where booster master cylinder will have a shallow hole

[tdangle]

I have a kit ordered for my 70 C20, but didn't buy the master cylinder/pv/booster they offered because it doesn't seem to be correct for the C20. In my research 71,72 C20's used a dual chamber 11" booster and master cylinder with a 1 1/4" bore. The rear portion of the m/c goes to the front discs and has a larger fluid capacity. the front goes to the rear brakes. This is opposite of a C10. From what I could tell the proportioning valve is the same for both C10 and C20 in years 71,72.