Brake lines at MC

[C3pilot]

Kind of embarrassed but I’m drawing a blank on which chamber of the master cylinder feeds which brakes. I have a ‘72 LWB 1/2 ton with power disc/drums. Got the truck a few days ago as a bit of a basket case and the brake lines have been pulled and bent/damaged.

I searched and have seen with different brake line routing at the MC, front chamber to the front of portioning valve and rear to the rear of PV. But…I’ve also seen it the opposite with the front plumed to the rear port of the PV.

Any input would be awesome!

[jimijam00]

Front to front, rear to rear.

[C3pilot]

Here’s a pic

[twoskies]

mine, might help

[C3pilot]

Thank you!

[C3pilot]

Another quick question on the 2 lines between the MC and PV. Are the same diameter? The rear one seems to be larger on mine. Didn’t know if there is a specific reason for this. Thx

[jimijam00]

3/16" front, 1/4" rear.

[Caddylackn]

The larger diameter line should come from the larger reservoir side of the M/C, which should be the feed to the side that goes to the drum brakes.

[jimijam00]

What if his MC is like the ones pictured above? Or what if he has an aftermarket one that has a larger front reservoir, like I do?

[dmjlambert]

I believe it is just the opposite, disc brakes in front get the larger reservoir, drum brakes get small reservoir.

[C3pilot]

Exactly?!?! That’s what had be baffled. My MC reservoirs are the same size. I remember the disc having the larger reservoir. Either way I’m hooking the front to the front disc. IIRC the front chamber will be the last to fail as there’s only 1 spring on one side of that port ( or some kind of safety feature design incorporated in the design). Something having to do with when the MC is fully compressed, HEAVY brake pedal pressure, all springs are compressed and there is a direct pushrod connection to the front MC chamber.

Not explaining it well but m under the truck as I type!!
Thx again for the help!

[C3pilot]

Refreshed (not really has the old internals). But it looks better!!

[68 P.O.S.]

You have a newer style MC used for Disc/Drum or Disc/Disc setups. It will work fine on your truck. And yes, C10 lines are front to front, rear to rear. C20s are the opposite...weird.

[Caddylackn]

Your older Master Cylinder can be used for disc/drum set ups but the pressure residual valve (PRV) should be removed from the output side of the Master Cylinder that goes to the disc brakes. You can feel the PRV if you insert a small screw driver into hole (cone) where your brake line screws into. You can usually thread a wood screw into the end of the cone covering the PRV then use a claw hammer to pull the cone out. There is a spring and a rubber valve in there that can be taken out then the cone cleaned up and replaced without the valve or spring.

The 1/4" line should go to the drum brake side and the 3/16" line should go to the disc side. It is okay to keep the stock 1/4" line that splits and feeds the new disc brakes but it is not okay to run the 3/16" line to the drums. Drum brakes require a larger brake line and a PRV to operate properly. It takes more fluid and pedal travel to operate drum brakes since the wheel cylinder pistons (4 of them) travel farther and use more brake fluid volume than the 2 caliper pistons. The PRV keeps around 6 psi in the rear brake system to take up all the slack out of the mechanical linkage to minimize pedal travel (and brake fluid volume) required to operate the drum brakes. You don't want to run the stock 5-6psi PRV on disc brakes because it could create brake drag. If you don't have drag running the PRV on the calipers then I guess you can leave it in.

[C3pilot]

Originally Posted by Caddylackn

Your older Master Cylinder can be used for disc/drum set ups but the pressure residual valve (PRV) should be removed from the output side of the Master Cylinder that goes to the disc brakes. You can feel the PRV if you insert a small screw driver into hole (cone) where your brake line screws into. You can usually thread a wood screw into the end of the cone covering the PRV then use a claw hammer to pull the cone out. There is a spring and a rubber valve in there that can be taken out then the cone cleaned up and replaced without the valve or spring.

The 1/4" line should go to the drum brake side and the 3/16" line should go to the disc side. It is okay to keep the stock 1/4" line that splits and feeds the new disc brakes but it is not okay to run the 3/16" line to the drums. Drum brakes require a larger brake line and a PRV to operate properly. It takes more fluid and pedal travel to operate drum brakes since the wheel cylinder pistons (4 of them) travel farther and use more brake fluid volume than the 2 caliper pistons. The PRV keeps around 6 psi in the rear brake system to take up all the slack out of the mechanical linkage to minimize pedal travel (and brake fluid volume) required to operate the drum brakes. You don't want to run the stock 5-6psi PRV on disc brakes because it could create brake drag. If you don't have drag running the PRV on the calipers then I guess you can leave it in.

Thanks for the heads up! I’ll check to see if there’s a PRV on there. I believe this was a factory front disc truck so maybe this MC is good to go.

Thx!