Bleeding Brake Lines

I bought braided brake lines from Aircraft Specialty, which arrived this week. Quality looks excellent. So I?m planning the installation and can?t find any information on procedure for bleeding the lines. I?m looking at the plans for the brake system and really have two questions?

The brake system from the pilot?s master cylinders to the calipers is a closed system and not open to the reservoir. How do I bleed air and/or introduce new fluid? I can pump brake fluid up through the bleeder screw at the caliper but if the top end of the system is closed where is the air bubble going to go?

Second question is how to bleed the crossover lines from the passenger brake pedal cylinders to the pilot?s master cylinders with the lines essentially being a closed loop? The passenger side brakes are a little spongy and I think there must be some air in the crossover lines.

Get a good hand oil pump and bleed from the bottom of the calipers up. You may need to disconnect the master cylinders from the pedal (at the top) to completely open the valves. Make sure to use an overflow can at the reservoir. It takes some patience. We have bled ours three times since completetion and use that procedure every time.

Jim,

Check your email for a message from me on this topic.

Get a 1 gal sprayer from a farm supply store ($9.00 or so), cut the spray nozzle off and slide 5/16" id clear tubing onto spray nozzle shaft after trigger valve. On the other end of the tubing transition to correct size tubing to fit over brake bleeder fitting. Put brake fluid into sprayer tank..pump up (have a helper work valve and keep tank pumped up while you slowly pump brakes. One side at a time (you will probably have to do bleeding on each side 2 times) as bubbles will show up after the first time.

I used a 1/4" barb fitting on reservoir tank w/tubing to catch overflow...

Piper J3 said:

.......The brake system from the pilot?s master cylinders to the calipers is a closed system and not open to the reservoir........

Click to expand...

Jim, if that is really true then something was assembled incorrectly in your airplane. Since you have found a few other things that are a bit odd about your particular craft I wouldn't count out the possibility that the brake lines are mis-routed.

Jim----we generally bleed from calipers up. It will take a pretty good volume of fluid to fill everything the first time.
Tom

And, if you pump up from the bottom, and no fluid comes out of the resivior, something is hooked up wrong, as I suggested in my earlier post. Also there are other ways to pump fluid in at the calipers. For example, I used a large pump-type oil can. Because of the loops in the tubes up by the brake pedals, it can be very hard to get all the air bubbles out. I remember having to loosen the connections on the master cylinders to let the air escape.

part of my procedure is to fill the caliper before pumping from the bottom up.

when disconnecting the caliper for maintenence i cap off the line so as not to loose the fluid from above.

tricks from turbo

regardless of the method, check for air bubbles in the clear lines under the panel. It can be a real pain, and often requires pumping the cylinders some and perhaps loosening some lines near bubbles to get the air out. These are the hardest to bleed of any I have encountered.

Asking how to bleed brakes is like asking whether you should use primer! Everyone has their own way of doing it. Fact is, there are many ways to accomplish the bleed and all will work, just some are easier than others.

First and foremost, all the bleeding in the world is not going to get the air out if your system has an air leak somewhere. Common culprits are the fittings that screw into the side of the brake cylinders. This is a pipe thread so make sure there's thread sealant on them. I personally would not recommend loosening any fittings to let air out except the bleed fitting on the caliper.

Second, I just haven't had much luck bleeding from the bottom. Doing so doesn't allow me to use the pedals to push brake fluid rapidly through the system. Maybe I just don't have the patience for it.

Third, there's usually a little bubble of air at the top of the cylinders that's tough to get out. On the -12, I can tilt the pedals back while pumping them to get it. On the -6, I had to pull some bolts on the pedal assembly to get the cylinders to lay flat enough. On the Rocket, I had the reservoirs screwed directly to the pedals. They we IMPOSSIBLE to bleed. Learned my lesson there.

Finally, I've found it best to have a completely closed system. Whatever pumping method, whatever tools used to circulate the fluid, all hose ends should be in fluid and not exposed to the air where they can suck air in. I bought a cheap bleeding kit at the auto parts store for the fittings and hoses.

Of course, wear gloves and if you're really brave, use Mobil 1 ATF instead of the other nasty stuff. I've used it for years, and I don't feel bad loading up cups for the bleed and then throwing it out.

I built an Aventuta HP that used ATF as specified by the brake manufacturer. I like the idea, but I don't know enough about the compatibility with brake hose, seals, gaskets, etc. in the Matco brakes.