3rd line coming out of master cylinder?

jakethesnake06 · Apr 24, 2026

I recently got a 77’ c30 as my first project, so I am new and learning. I am currently working on the brakes. Can some please explain what this 3rd smaller brake line is that taps off one of the two larger main lines and goes into the firewall it appears. I assume it is for some monitoring system?

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Turbo4whl · Apr 25, 2026

That line is for a trailer electric brake controller.

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jakethesnake06 · Apr 25, 2026

Oh I see, that makes sense. Thank you!

Radiohead · Apr 25, 2026

Just curious. Does it terminate to a controller or does it dead head somewhere?

Turbo4whl · Apr 25, 2026

Radiohead said:
Just curious. Does it terminate to a controller or does it dead head somewhere?

Yes, hydraulic line just to the controller. Years before electronic brake controllers, this was the way to power trailer electric brakes. Small cylinder pushes the lever over. The more it moves, the contact moves down the resister until full power. You can fine tune by spinning the lever so it applies faster or slower.

Many times an added external resister which makes for the fine adjustments for the trailer braking. You can apply the trailer brake only by moving the lever over, or hold it off while backing up.

Turbo4whl · Apr 25, 2026

@jakethesnake06

...an added external resister, may look like this:

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75gmck25 · Apr 26, 2026

That type of trailer brake controller has been illegal for several years because any leak in that extra line could make the primary brakes ineffective.

Most newer controllers either operate using a switch on the brake pedal, or they have an inertia switch with a pendulum inside that swings and applies the trailer brakes as you brake the truck. All of them have some type of sensitivity adjustment.

A fully wired 7 pin trailer connector will have a trailer brake wire that runs to the trailer plug. When the controller sends a signal down that wire it uses a battery on the trailer to apply electric brakes.

FYI- don’t be confused by U-Haul trailers, which use a braking switch in the trailer tongue. It compresses/slides and applies the trailer brakes when the trailer weight pushes forward against the tongue.

CountKrunk · Apr 26, 2026

Whats the little slidy knob do? My truck has one, and the knob slides back?

Its disconnected, so maybe that's why.. going to remove it when i dig into the cab.

bucket · Apr 26, 2026

75gmck25 said:
That type of trailer brake controller has been illegal for several years because any leak in that extra line could make the primary brakes ineffective.

Most newer controllers either operate using a switch on the brake pedal, or they have an inertia switch with a pendulum inside that swings and applies the trailer brakes as you brake the truck. All of them have some type of sensitivity adjustment.

A fully wired 7 pin trailer connector will have a trailer brake wire that runs to the trailer plug. When the controller sends a signal down that wire it uses a battery on the trailer to apply electric brakes.

FYI- don’t be confused by U-Haul trailers, which use a braking switch in the trailer tongue. It compresses/slides and applies the trailer brakes when the trailer weight pushes forward against the tongue.

There was another thread about this. I believe them being outlawed is a false statement? I'd rather not find that thread and link it, because it kind of turned into a sh!t show with an "expert" in the feild being disproven.

PrairieDrifter · Apr 26, 2026

75gmck25 said:
That type of trailer brake controller has been illegal for several years because any leak in that extra line could make the primary brakes ineffective.

Most newer controllers either operate using a switch on the brake pedal, or they have an inertia switch with a pendulum inside that swings and applies the trailer brakes as you brake the truck. All of them have some type of sensitivity adjustment.

A fully wired 7 pin trailer connector will have a trailer brake wire that runs to the trailer plug. When the controller sends a signal down that wire it uses a battery on the trailer to apply electric brakes.

FYI- don’t be confused by U-Haul trailers, which use a braking switch in the trailer tongue. It compresses/slides and applies the trailer brakes when the trailer weight pushes forward against the tongue.

They aren't illegal.

It's no different than any other part of the brake system leaking.

PrairieDrifter · Apr 26, 2026

CountKrunk said:
Whats the little slidy knob do? My truck has one, and the knob slides back?

Its disconnected, so maybe that's why.. going to remove it when i dig into the cab.

It's to apply the trailer brakes manually, it is also progressive. Farther you move it the harder the trailer brakes come on.

Sad Sack · Apr 26, 2026

PrairieDrifter said:
It's to apply the trailer brakes manually, it is also progressive. Farther you move it the harder the trailer brakes come on.

All in all good info, never messed with brake controllers so any info is good info.

Turbo4whl · Apr 26, 2026

75gmck25 said:
That type of trailer brake controller has been illegal for several years because any leak in that extra line could make the primary brakes ineffective.

^^^This is simply not true.

75gmck25 said:
A fully wired 7 pin trailer connector will have a trailer brake wire that runs to the trailer plug. When the controller sends a signal down that wire it uses a battery on the trailer to apply electric brakes.

^^^ This is also wrong! Current from the towing vehicle brake controller through the trailer plug direct to the trailer brakes.

Yes, heavy trailers have a battery. This is for the trailer brake-a-way system. Should the trailer become disconnected, the brake-a-way switch applies the trailer battery to stop the trailer.

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Turbo4whl · Apr 26, 2026

bucket said:
There was another thread about this. I believe them being outlawed is a false statement? I'd rather not find that thread and link it, because it kind of turned into a sh!t show with an "expert" in the field being disproved.

Some states may have different laws for trailers. In Pennsylvania every heavy trailer wheel set are required to have brakes. Some states may only require one axle, not both.

When the forum members are talking about fuel injection and other items I don't know enough about, I keep quiet. When wrong info is posted about safety items, I'm ready for the Sh!t Show!

75gmck25 · Apr 26, 2026

I did not make a serious attempt to find out if connecting hydraulic lines to your master cylinder is still “legal”. However, in some states you may have trouble during an inspection if they notice it. It just depends on the inspector. However, there are definitely now newer and better designs to sense vehicle braking and send a signal to the trailer brakes.

I was also too fast in wording my answer about the brake controller wiring to the 7 pin plug. The brake circuit activates the trailer brakes, and “may” ( or maybe not) also be used charge a battery for the break-away system.

3rd line coming out of master cylinder?

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