brake air bubbles appearing

We're running into a problem with air bubbles in the brake lines of an RV-7a. There was definitely a leak in the connector between the soft line from the pilot's seat right brake to the aluminum tubing out to the wheel. We didn't notice the leak for a while and managed to let the reservoir empty and got air bubbles in both brake lines (at least that's what we think happened), leaving the co-pilot's brakes very spongy and not so useable.

Fixed that leak, rebled the brake lines and the right brake now seems to have the right amount of stiffness and appears to work well. However, we're running into an odd problem with the left brake. If the co-pilot's left brake pedal is pushed, an air bubble will show up in the line from the master cylinder to the reservoir, just outside of the master cylinder. If the pilot's left brake is pumped a number of times (20+ times), a bubble appears in the line between the pilot and co-pilot master cylinders, right after the pilot's master cylinder. We cleaned up the cockpit after the original leak and can't find any brake fluid leaking anywhere. We rebled the left line a couple of times, and ran into the exact same behavior each time.

Neither right brake is "spongy", but we're worried about these air bubbles and having to do this whole process after another couple of flights. The plane has about 60 hours on it, brake line design is according to Van's designs. Any suggestions on what could be going on or how worried to be about the air bubbles?

Thanks,

Brian

I had bubbles visible that simply would not go away. Even so, the brakes were always plenty firm and functional. Still, it looked just...wrong. So, to eliminate all those clear runs of tubing, I removed them entirely and installed dedicated fluid reservoirs on each master cylinder.

Brakes and bubbles

OK, let me start off by saying I'm NOT anything approaching a brake expert. I have redone my brakes on my -8 once.

I don't know if this is your problem, but after I let my brakes go dry due to a leak in what sounds like the same area, one of the master cylinders leaked internally (I think the seals dried out and wouldn't do their job afterwards). With only the single brakes in the -8, it simply manifested itself as a very spongy brake. Maybe it could be allowing air into the other side on yours?

John
RV-8 N94DW

Two weeks ago, I did the process, where you take a cheap quart size lawn sprayer from Lowe's or Home Depot, that has a hand pump for air pressure.

Attach the pump along with brake fluid to the brake fitting at the wheel, and another clear tube from the reservoir into an empty bottle.

This will easily push plenty of fluid through the total brake system, and every last bubble along with it, as you'll see them come through the line out of the reservoir into the bottle. Doesn't matter if there are high points in the lines or not. You don't touch the pedals either.

Can be done with a hand oiler too, but with the air pump, this is an easy one person job. --repeat for the other side--

Pump sprayer is about $7.00

L.Adamson

We've been using the oiler hooked up to the bleed valve method to rebleed the lines, and that appears to be working pretty well. We have an adapter and a 1/4" nylon line we attach to the reservoir to prevent a huge mess in front of the firewall. Now that we have way too much practice at bleeding the brakes, we can actually do it with very little mess. So at least we've learned something along the way .

We were beginning to wonder about the co-pilot left master cylinder and whether it was bad. We were going to replace it, then noticed the bubbles appear after many uses on the pilot's left master cylinder and wondered if there might be something else. I'm sure the co-pilot left master cylinder was dry for a little while, but I don't think the pilot's left master cylinder ever was.

Me Too.

I had a similar problem last week.
I have dual brakes in my RV7.
I rarely fly from the right seat, but did last Week and found the RIGHT brake sponge and subsequently found air bubbles in the Line between the First Officer's RIGHT Brake and the Capts RIGHT brake.
With a friend we bled the RIGHT brake from the right wheel UP, until the bubbles were gone and reservoir was full.
When I taxied out I found I had almost NO RIGHT brake.
So we taxied back and rebled it and as we did, I then noticed air coming UP the line from the brake, (ie. from the Capt's RIGHT brake master) so I place a container under the Brake Reservoir (Mine is on the Pedals like Brian's) and told my mate to just "Keep on Pumping".
Eventually, after pumping through a cupful of brake fluid, all the Air was expelled and the brakes were firm.
I suspect that on the first Bleed, my mate did not have the tube firmly on the bleed screw and inject some air with the brake fluid. Whilst I am not suggesting that is cause of your problem, I am sure that some bubblles do become trapped and putting an excess fluid through the sytem is the only way to purge the bubbles.
SO, might try (if you haven't already) pumping a good cupful through the system, as per L.Adamson's method.

It turns out we just didn't try enough times. Apparently master cylinders when back-filled can catch a couple bubbles in them, while letting fluid flow right by and into the reservoir. The machinist who clued us in to this phenomenon said on the large machines he used to work on, they would create a loop and run the cylinder to push the bubbles out of the system. We just worked the brake cylinder, letting a bubble form on the low pressure side, then push the bubble into the reservoir by using the oiler can at the brake trick. Repeat 4-5 times for the pilot then 6-7 times for the co-pilot left master cylinders and it appears the brakes are good to go again. Thankfully, it appears we didn't ruin the co-pilot master cylinder by letting it dry out.

Thanks for all the advice!

Brian

If you have a parking brake made with one master cyl. like on Pipers hand brake, then you just push a clear line on the bleeder of (one at a time) the wheel cyl. run the line to the res. have it 3/4 full. Run the line so you can see it from inside the A/C. Then just get in and pump the parking brake, watch the clear line and when here are no bubbles left, then you are done. WHAT no parking brake? how will you get out of the plane on a slope or when the wind is up......