Air in brake lines

[Jeff Vaughan]

Need some help here. I have filled the brake lines from the bleeder nipple with a plant sprayer set up that others have used and suggested. I still get air in the lines. Is there a trick here?

[Christopher Murphy]

if your brake cylinders are mounted upsidedown like they are on the rv-4 you will have to take them off and turn them right side up to get all of the air out. If they are mounted right side up then you might have a loose connection somewhere. Pumping the fluid up from the bottom sounds great in theory but if that is not working fill the resevoir from the top and do it the old fashioned way and bleed the brakes by forcing the air out the bleeder valve. Be sure to open bleeder,apply the brake and hold it down,close bleeder and repeat. You will need a helper for this.

Chris Murphy A&P RV-4 Builder

[f1rocket]

I have been "around the block" on this one. Here's what works for me and I've tried just about everything.

First, I bought a bleeder system off of eBay which is nothing more than a container and hoses with a rubber bulb to pump the fluid. Here's how I do it:

1. Fill reservoir with fluid right up to the top.
2. Connect the feed line to the top of the reservoir.
3. Connect the bleed valve to the return line and insert the return line into the brake fluid container, making sure the end of the line is buried in the fluid so no air can enter.
4. Open bleed valve a couple of turns.
5. Pump fluid through the system with the bulb until no more air comes out the bleed valve line.
6. Vigoriously pump the brakes making sure to pull the brakes all the way back to release the valve in the brake cylinder. It's that last 1/8th of an inch that opens the vent in the cylinder.
7. Close the bleeder valve.
8. Disconnect all the hoses and stuff.

This works for me. YMMV.

[Mike S]

Originally Posted by f1rocket

making sure to pull the brakes all the way back to release the valve in the brake cylinder. It's that last 1/8th of an inch that opens the vent in the cylinder.

Not trying to hi-jack this thread or anything, but you have just hit on one of the major problem areas in these systems.

Dragging brakes, early pad wear, etc all result from that last 1/8".

The dynamics of the spring rate are such that the spring is weakest where it is most needed.

This is why it is so important to have free, properly aligned brake pedal installations.

Booster springs, and long pivot bolts, ETC are just band aids for this basic problem.

[f1rocket]

Totally agree. The problem on the Rocket is even worse because the geometry is all jacked up. I'm in the process of adding springs too since I occasionally have a problem with the brakes dragging. Good point.

[N941WR]

Jeff,

I bought a chemical sprayer and like you, I had a problem with air in the lines. It turns out the problem I had was the sprayers pump sat below the level of fluid. Every time I would pressurize the tank, it would pump air into the fluid. Not good.

The solution was to buy an identical but deeper (taller) tank and use the pump from the smaller tank. This kept the pump up, out of the fluid. Problem solved.

Also, I used the plastic brake lines in the fuselage. This allowed me to see any air bubbles that might be trapped in the crossover tube. It took a while but eventually I was able to purge all the air bubbles.

[Jeff Vaughan]

Thanks everyone for your help