Master Cyl and Brake Question
 

On my 1978 K30, I have a switch called a "Mico Dualock" mounted on the dash. I googled that and it seems to be a supplemental brake system. Then I have what I think is an electrical trailer brake setup hanging under the dash.

I have a lot of "extra" brake stuff under the hood, which I am guessing is part of the Dualock. There are extra brake lines that have been tapped into the master cylinder lines and they run to something else (pics below) on the inner fender, a vacuum line (i think) leaves the fender and heads under the dash.

This is a flat bed farm truck that does farm type work and it always fun to drive something with a 454 in it. I am not a commercial hauler and the extent of my hauling is fence post, hay, cars, tractors, and cattle trailers.

Anybody know other than additional braking, why a 1 ton would have this kind of brake system?

I would like to remove it completely, seems to be disabled / non-working anyway. Other that bleeding the Master Cyl and wheel cylinders again, is there a downside to removing it?

It that box hanging under the dash with the black knob an electric brake system for trailers?

Thanks!

Re: Master Cyl and Brake Question
 

It appears to be a parking brake. Rather than just the rear drums pulled by their cables, it holds hydraulic pressure to all four wheels. Looks to be vacuum operated, I'm guessing those taps at the master cylinder are some kind of hydraulic valve that blocks the lines and holds the pressure to the brakes.

What it's useful for, I'm not sure. Maybe if you park on hills a lot with a really heavy load on it, or a trailer attached? It looks like it's fairly easy to remove and return the system to the stock configuration if you wanted to though I can't tell if the stock lines were hacked up to install it. Have you tested it to see if it works?

Re: Master Cyl and Brake Question
 

it would come in hany when loading trailer when the back tires come off the ground and start sliding

Re: Master Cyl and Brake Question
 

Yes the box hanging under the dash is for electric trailer brakes.

Whats the knob to the upper left of the brake knob for?

Re: Master Cyl and Brake Question
 

No idea what that knob is supposed to do, right now it does nothing :) I am going to pull the dash this weekend and start tracing wires and hoses. I was hoping that box under the dash was for electric brakes. Will see if I can follow it's wires to the back.

Re: Master Cyl and Brake Question
 

When I flip the switch, I get a humming sound from under the hood. Tonight or this weekend I will have someone flip that switch while I am watching so I can see what is happening. Other than the noise it seems to be inop, I can put it in drive and pull forward.

Looks like the only line or Master Cyl change is a fitting between the MC and the brake line, that fitting has a port that another line comes off of. I think I can remove the fitting and put the brake line back in it's original spot, I hope!

Re: Master Cyl and Brake Question
 

I'm thinking they way it would work is with the engine running, you press the brake pedal down fully and then flip the switch to the on position, and that should hold pressure in the lines.

If it doesn't work, make sure it's the system is still hooked to an engine vacuum source, as it may have been disconnected in the past by someone who didn't know what it was.

Re: Master Cyl and Brake Question
 

Apparently in order to set it you need to have the engine running, put it in park, set mechanical p-brake, take your foot off the brake pedal, switch the dualock onto full lock(some kind of alarm should go off), depress your brake pedal till alarm goes off.
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Re: Master Cyl and Brake Question
 

i have this on my tow truck and love it. with it running press the brakes flip the switch and relaese and it will lock all four wheels. if you are doing any kind of trailer loading or winching it is a must have i think

Re: Master Cyl and Brake Question
 

I got it fixed and it works like a charm. That "Mico Dualock" switch on the dash had three vacuum lines attached to it, at least they were supposed to be attached! One hose was disconnected and one was broken. Two of the lines ran to the vacuum port that attach to the master cylinder and the other was attached to the intake manifold. As an added benefit, fixing that massive vacuum leak made it run better :)

The way to operate it: With the engine running (in Park), press the brake pedal, turn the knob to "Lock", and release the brake pedal. Done, now all four tires are locked. Engine has to stay running.

Thanks!