Hello I am having a problem with my brakes, replaced all rings in master cylinder s , blead both lft and rt brakes with no air coming out. There are no leaks . And when I pressurized the whole system. There where no leakers . I have good solid brakes on the ground before take off. I put the gear up and as I put the gear up pump the brakes. The peddle will slowly go down to about half the peddle . Blead the system when I landed and no air came out. Any suggestions.

Don

We had the same problem on a twin Comanche last fall. After doing all of the stuff you did we were looking at the brake assemblies and measured the brake disc thickness, even though they looked good. The discs were worn beyond minimum thickness. We replaced them and the problem was solved.

 

Don,

Earlier this year I did a landing gear complete overhaul and in the process had to deal with brake bleeding. I was warned that it is very hard to bleed the brakes and quite possible to have air trapped in points that is difficult to get out. Some advised me that the best way to bleed the brakes is from the bottom up. In other-words instead of adding fluid to the reservoir and bleeding at the caliper you do it the other way around. You get a pressure bleeder to push the fluid up from the caliper to the reservoir (after all air bubbles naturally want to rise anyway).

This approach worked for me the first time. I definitely had air in the lines because my brake lines were completely apart. I had access to a pressure bleeder with the correct Hydraulic fluid (Mil-5606 - Royco 756A). At a local hardware store you can get a cork to use as a stopper for the top of the reservoir and make a hole in the top to fit a tight tube for draining. Then we simply pumped the 5606 up from the bleeder valve on the brake caliper until the fluid coming out the tube in the reservoir no longer showed any air bubbles. Using this approach I've had no problems at all. I would try bleeding using this method before replacing parts.

- Charles

We always bleed from the bottom up using a pressure bleeder with enough pressure and volume to push any air bubbles through the high spots into the reservoir. Siphon most of the fluid out of the reservoir before bleeding and fill the reservoir from the bottom bleed. The reservoir holds more fluid than the bake calipers and lines.

Jim

 

I have learned a few things about brakes on two restorations I have done on a 172 and comanche 250. I hope this will help others with soft pedals.

I struggled terribly trying to get good pedal on my 172 while pumping fluid up from caliper. The problem was, the cap on the resivoir is behind the brake pedals. Peeking over the pedals while pumping the can with my foot (working by myself) I accidently pulled back on the pedal and got a face full of fluid. Just the weight of the top of the pedal on the mechanism/cylinder was keeping the cylinder partially closed or pedal applied. This was fresh on my memory when doing the brakes on the comanche restoration.

Step one: use a couple small bungie cords to gently pull back on the top of the pedal to fully extend the cylinders.

Step two: use a gallon can of fluid, a small electric pump ( like a r/c model fuel pump) and several feet of clear tubing to go from the gallon can to the pump to caliper bleeder, and return from the nipple on the cap of the airplane brake resivoir. With a pick up tube in the gallon can, and the return line in the same can you can run the pump and observe the air bubbles purge out till you see a solid stream with no bubbles. This has worked great for me, and provided a firm pedal right on the top of travel of the toe brakes.

Hope this will help someone with this problem