Plastic Line with Brake Fluid by Electric Boost Pump

[Lima Whiskey]

The other day we removed the electric boost pump in our 1980 M20J. To access it, we removed the forward left access panel - right where the oil breather vent comes out. Just aft of the left cowl flap. When we removed the panel, we found a plastic line filled with some brake fluid which was separated from a t connector that had two braided cables running into each side of the t connector. 
 

it appears that the compression connection that held the plastic line let loose. 
 

Does anyone know what this line does and what needs to be done to reconnect it so it doesn’t happen in the future. 
 

We have not experienced anything negative with the brakes.

 

 

[EricJ]

Maybe the line for the parking brake?   I'm guessing everything is capped off there?   Otherwise you'd have fluid going everywhere and no brakes.   The parking brake valves can be problematic and it wouldn't surprise me if somebody capped it off.    

[PT20J]

52 minutes ago, Lima Whiskey said:

The other day we removed the electric boost pump in our 1980 M20J. To access it, we removed the forward left access panel - right where the oil breather vent comes out. Just aft of the left cowl flap. When we removed the panel, we found a plastic line filled with some brake fluid which was separated from a t connector that had two braided cables running into each side of the t connector. 
 

it appears that the compression connection that held the plastic line let loose. 
 

Does anyone know what this line does and what needs to be done to reconnect it so it doesn’t happen in the future. 
 

We have not experienced anything negative with the brakes.

 

 

That is the line from the brake fluid reservoir in the tail. The lines from the T feed the master cylinders. Just reattach it and tighten the compression fitting and give it a pull test. You’ll probably need to bleed the brakes since air will have gotten into the lines.

[N201MKTurbo]

https://www.aircraftspruce.com/menus/ha/fit_znylo.html
 

[Lima Whiskey]

6 hours ago, EricJ said:

Maybe the line for the parking brake?   I'm guessing everything is capped off there?   Otherwise you'd have fluid going everywhere and no brakes.   The parking brake valves can be problematic and it wouldn't surprise me if somebody capped it off.    

 

5 hours ago, PT20J said:

That is the line from the brake fluid reservoir in the tail. The lines from the T feed the master cylinders. Just reattach it and tighten the compression fitting and give it a pull test. You’ll probably need to bleed the brakes since air will have gotten into the lines.

This is very helpful.  I thought it might becoming from the fluid reservoir. I don't know anything about brake lines. How does the brake system actually draw the fluid from the reservoir?

[PT20J]

39 minutes ago, Lima Whiskey said:

How does the brake system actually draw the fluid from the reservoir?

Gravity. The reservoir is the highest component of the hydraulic system. 

[N201MKTurbo]

15 hours ago, PT20J said:

Gravity. The reservoir is the highest component of the hydraulic system. 

 When foot pressure is released on the master cylinder, a spring pushes the plunger back. If the master cylinder isn’t completely full, the spring causes a low pressure that is lower than ambient pressure. This pressure differential is what causes the fluid to flow from the reservoir to the master  cylinder. 
 

The reservoir could actually be somewhat below the master cylinder and still work. As long as weight of a column of brake fluid was less than atmospheric pressure.

The reason the reservoir is the highest point in the system is because air bubbles are less dense than brake fluid, therefore they will tend to migrate towards the highest point in the system where the air will eventually go out the vent.

[Jim F]

This week I had a similar problem but with the bulkhead fitting 4 ft aft.  After 44 years the line pulled out of the forward side of the bulkhead fitting.  As PT20J states, this is the line that heads aft to the reservoir in the tail access panel.  When the brake pads wear, and the brake pucks move out a bit there is more hydraulic fluid in the caliper.  The master cylinders will pull hydraulic fluid from this line and the reservoir.  If air gets into the system, it's a challenge to get out.  When you reconnect the line, you can try to push hydraulic fluid back to the reservoir by pushing in one or both caliper pucks.  this will push fluid back to the reservoir.  On my K the slope back to the reservoir is not all uphill so I need to work it a bit.  I will put a slight vacuum on the reservoir plug fitting to pull the air into the reservoir. My guess is that the line shrunk a bit, and the factory ty-wraps were too tight, and the line slid out of the bulkhead fitting.  

Jim