Dual Master Cylinder

[j mccormick]

If you install a dual master cylinder on your M37 and retain the drum brakes, and if the master cylinder is made for a disc brake application you will need to install a residual valve in each of the lines to prevent loss of pedal when you let the truck sit for awhile. The valves are available from Speedway Motors in Lincoln, NE.

[w30bob]

Hi J,

You'd need a proportioning valve too. In regards to the residual valve, you need on in each circuit, so we're talking one for the front brake circuit and one for the rear brake circuit. So which valve do you use.....the 10 psi valve is designed for civilian brake applications and it's function is to overcome the return spring force the shoes put on the wheel cylinder. But the return springs on an M37 are perty big. I'm not sure 10psi would do the trick. Anybody done this successfully?

regards,
bob

[achiem37]

The M37 that I had I installed a dual master cylinder. I ran it for a year and found the first pedal application felt a little soft. I installed the 10 psi residual pressure valve for the rear stock drum brakes and a 2 psi residual valve for the front discs. The pedal stayed firm and never felt soft again. 10 psi at the wheel cylinder seems to work just fine on my truck for the drum brakes. Which master are you using?. I used a napa # 39017 for my manual brakes. It was a fair bit of pedal effort to stop the truck though and it has the 1 5/16" bore that you need for the volume of the drum brakes wheel cylinders. Terry

[j mccormick]

I installed the 10 psi residual valves in both the front and rear lines since I still have the drum brakes all the way around. I don't need a proportioning valve with the front not being disc. I got a late 70's Gm master cylinder from NAPA with a 1/4 inch bore- same as the stock M37 bore, The pedal stays firm now.

[jimmy_stikx]

Where are these residual valves installed? I'm thinking of switching to a dual setup on mine, it sat for 2 years and the brakes went kaput.

[j mccormick]

Just install them into the brake lines, one in the front brake line and one in the rear brake line. I put mine near the master cylinder itself but I think anywhere in the line would be okay. They just keep a residual pressure to keep the fluid from rushing back into the master cylinder and letting air in. The valves I bought had a 3/8 pipe thread in them so I got adapters to fit the 1/4 inch line with the inverted flare line on the M37. The brakes used to be hard to bleed without the residual valves, but after installing them I was able to bleed them by myself by cracking the bleeder screw at the wheel cylinders slightly and slowly pushing the pedal to get the air out.

Joe

[xd2200]

What was the NAPA part # for the MC? Was it bolt-in or did you have to do some fabrication/modifications?

[jimmy_stikx]

I ordered mine from Speedway motors.

[j mccormick]

The NAPA part number was M2085, a new master cylinder, for a 78-80 K20 GM truck. It has a 1 1/4 inch bore like the original M37 bore. You do have to make some holes in the frame for the lines to go through, and maybe modify the mounting holes, its been awhile since I did it but it was an easy change over.

[jimmy_stikx]

What kind of adapters are going to be needed? NAPA is a good distance from me, so kinda want to get all the parts in one shot.

The M/C I have ordered has the ports on both sides, so hopefully no frame mods other than for the bolt pattern

[j mccormick]

You will need the adapters, and I don't know what size the ports on your master cylinder are, to plumb the 2 residual valves into the lines. The residual valves I have had a 1/8 inch pipe (female) on both ends. So you will have to adapt to your master cylinder, through the residual valves. then into the brake lines on the M37. Mine has 1/4 inch lines with the inverted double flare.

Joe

[billy]

The NAPA part number was M2085, a new master cylinder, for a 78-80 K20 GM truck. It has a 1 1/4 inch bore like the original M37 bore. You do have to make some holes in the frame for the lines to go through, and maybe modify the mounting holes, its been awhile since I did it but it was an easy change over.

i was just checking out my leaky looks brand new M/C and it looks like i can remove the current one and simply drill 2 holes for the replacement.
can i just reconnect the lines
install the residual valves
bleed
and drive?

[j mccormick]

After mounting the dual master cylinder, run one line to the front brakes, if they are still drum brakes install a residual valve in the line, then run a line to the rear installing a residual valve in it too. The tee where the lines are split for the single master cylinder is eliminated, and the front and rear lines can be accessed there. Then bleed and you should be good to go. I've found that if your pedal doesn't stay high after sitting for awhile its probably the wheel cylinders need replacing.