I was watching a show on the Velocity network about truck modifications, and I saw the following closeup of the trucks master cylinder and booster.

Truck Master Cylinder

What I don't know is why the designers put the loops in the brake lines coming out of the master cylinder? Everything on a car is there for a reason. I thought this might be for vibration, to give the lines some flex, but both ends of the line are secured to the same rigid body, so how much flex would you need?

Why are these loops in the brake lines coming from the master cylinder?

  • 2
    Interesting. Now I want to know. Commented May 24, 2016 at 17:32

5 Answers 5


It is about vibration. The master cylinder and brake booster are mounted to the firewall; the firewall flexes. They move up and down significantly as the body jerks up and down when driven over bumps. This solution is cheaper to implement than changes to the firewall. It would be challenging to build a firewall rigid enough to support that weight.

  • Oh! I didn't realize the firewall flexed on bumps. But then I am still looking to buy my first pickup truck. Thanks Fred, great answer!
    – cdunn
    Commented May 24, 2016 at 19:59
  • Useful info. Still seems like an excessive number of loops, though. Commented May 25, 2016 at 9:52

They are often times referred to as service loops. Their purpose is to allow some flexibility in the event the master cylinder has to be replaced, or a fitting needs to be repaired. It makes it easier to align the fittings without crossthreading them. AS @Fred Wilson has stated there is flexing. The brakelines will typically be anchored to the frame. The master cylinder is mounted to the body. The body is mounted to the frame via rubber body bushings. The bushings isolate the body from engine and road vibration. This allows the whole body to move, which could stress the lines if they where run straight to the mastercylinder.

  • So it's for both, very cool. Thank you for the great answer!
    – cdunn
    Commented May 24, 2016 at 20:41

Since it is a modified truck, obviously new brake lines and an old Master Cylinder, the answer is obvious.

The truck modifier purchased standard-length brake lines, and used the loops to take up excess length, rather than cut off the flared ends and re-flare them.

How do I know this? I've done it myself.

  • 3
    While this may be the case here, it does not explain why the loops would be used on cars from the factory.
    – rpmerf
    Commented May 25, 2016 at 12:59

The master cylinder was mounted on a fixture attached to the frame and the brakes were bled. As it went down the assembly line the body at some point was lowered on the chassis and the master cylinder was removed from the fixture and mounted to he cowl. The coiled lines allowed movement of the lines without linking or bending.


The loops are there to allow the flex between the body and the chassis on body on frame vehicles. You will notice that most if not all unit construction vehicles do not have these loops

  • Hi Matthew - please read existing answers first, so you don't repeat an answer.
    – Rory Alsop
    Commented Nov 1, 2021 at 13:12

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