Single- to dual-circuit conversion details?

[slantd100]

I'm swapping a 1968 dual master cylinder onto a 1965 D-100 with stock, unboosted drum brakes all around. I'll disconnect the rear brake line at the junction block nestled in the frame under the driver's feet, then use a union to connect it to the new line coming from the rear port of the new master cylinder. Then I'll plug the junction block where this line came out, leaving the lines to the front brakes and the line from the mc's front port intact. The rebuilt '68 mc that O'Reilly Auto Parts offers has a large reservoir at the rear and a small reservoir at the front. I'm planning on connecting the rear brakes to the rear (large) reservoir and the front brakes to the front (small) reservoir. Is that correct? Could someone with a '68 truck confirm this? Shouldn't the mc's reservoirs be identical in size? I thought that the asymmetrical reservoirs were for vehicles with disc brakes. Why the different-size reservoirs? The original '65 line from the mc to the junction block is larger than the other lines. I'm leaving that line connected to the junction block which sends smaller lines off to each front brake. I'm running a new smaller-size line from the mc to the union with the original small line for the rear brakes. Should that be the larger size, also?

[digdoug]

They sold you the wrong master. Like you said,both resevoirs should be the same size.The bigger one is for front disks. If you are planning on keeping 4 wheel drums,you might consider exchanging that mc for one for a 67or 68 d100.There are lots of ways to go about the plumbing,but getting a splitter/safety switch from a 67-71 d100 doner (since there are lots of parted out swepts in your area)might be easyest.

[RussRoth]

They sold you the wrong master. Like you said,both resevoirs should be the same size.The bigger one is for front disks. If you are planning on keeping 4 wheel drums,you might consider exchanging that mc for one for a 67or 68 d100.There are lots of ways to go about the plumbing,but getting a splitter/safety switch from a 67-71 d100 doner (since there are lots of parted out swepts in your area)might be easyest.

Just using a plain brass T is the easiest. All that switch does is turn on the dash light to tell you something is amiss with your brakes. Most of us would probably know that before the light even came on.

[65alive]

I'm not 100% sure if it's true for the sweptline trucks, but all of the "tees" I've seen in dual circut systems with the light switch built in are acctually a differential pressure vavle that not only turns on the warning lamp, but also shuts off the failed circut to retain the brake fliud for the circut still in opperation. They also have a little reset button on one end to recenter the vavle after the failed circut has been repaired.

[slantd100]

Since I didn't have a core, buying a rebuilt unit would cost me about $30 so instead of taking the rebuild they had on hand, I ordered a new unit for just $5 more. I'm curious to see if that one will have symmetrically sized reservoirs. I'm not concerned with using a pressure differential switch; I can just use the junction block, plug the port to the rear brakes and run a new line from the rear line to the mc. I haven't found a flare plug yet to plug that port. Is there any such thing as a flare plug? By the way, some of the master cylinders that I see online have a lid with a bolt between the two reservoirs instead of a bail. Is that a Mopar style or is it just an aftermarket variation? Those appear to have symmetrically sized reservoirs.

[Hobcobble]

I used part #4736221 from NAPA on 3 of my Sweptlines. The latest one was used on
my early '65 D100. I kept the stock line junction and plugged off the extra ports. I
went in with all new shoes, wheel cylinders, lines and added self adjusters all the
way around. There is quite a difference now from the old set up
John

[RussRoth]

I'm not 100% sure if it's true for the sweptline trucks, but all of the "tees" I've seen in dual circut systems with the light switch built in are acctually a differential pressure vavle that not only turns on the warning lamp, but also shuts off the failed circut to retain the brake fliud for the circut still in opperation. They also have a little reset button on one end to recenter the vavle after the failed circut has been repaired.

Sweptlines are not like that. It is basically just a T that turns on the light. Ask Wideblock(Trey). He finally took one apart like I did and discovered this to be the case. I believe you may be correct on some of the later models.

[nfury8]

Yip, the Swept blocks are just blocks. There are cut away drawings in the Service Manuals.

Flare fittings use straight threads, not pipe. So you could even plug them with a bolt
if desired.

[slantd100]

Thanks for the 4736221 NAPA number. I rejected the nonstock style offered by the other stores and ordered one of these OEM-look master cylinders today.

[Hobcobble]

Thanks for the 4736221 NAPA number. I rejected the nonstock style offered by the other stores and ordered one of these OEM-look master cylinders today.

John