Mushy brakes

[wslade2]

I've posted a few times about brakes as I had quite an issue with softness. Do not take anything for granted. You need to look at all components as these aircraft are getting long in the tooth. Like you, due to brake softness I started off with the swivels as they were clearly leaking. I had persistent issues with the toes going all the way to the floor during a prolonged run up. Ultimately this led to a complete system breakdown. I started with master cylinders. Of course be sure that the master cylinders are not scored or out of tolerance. In particular the piston uses something called a "lock-o-seal". Part of it is rubber which doesn't last forever. The gap on that has to be just right. Replaced. This improved but did not solve the problem so I started replacing lines, some were dry (rotting) some were gooey. There are flexible lines in the sidewall. You will have to remove side panels to inspect these and see if they may be leaking any. After I replaced some selected lines, and did the aforementioned items, 1 of the lines I did not replace (supposedly a lifetime silicone line), started new oozing at the crimp end. Lesson: There are no "lifetime" lines. For good measure ended up doing the parking brake valve. O rings were old. There could be cross pressure loss in the valve from one side to another. The technique for refilling the system has been reviewed here. I definitely used a pressure tank and a connector for back flushing from aircraft Spruce with a clear tube in the top of the master cylinder fill ports draining into a catch bottle. Yes you apply back pressured hydraulic fluid from the calipers forward to the master cylinders until there are no bubbles. For good measure I even cracked the lines in the sidewalls because they loop up and then down potentially holding an air bubble at the top of the loop. A dramatic improvement but still had softness despite all this. I had "rebuilt" the calipers (new o-rings) but one day I noticed stickiness around the calipers, one side worse than the other. It turns out that there was micro pitting corrosion in the sidewalls that was allowing some minor amount of pressure and fluid to escape at the caliper. Honing caliper did not work because the bore became enlarged with more leaking. This led to quite a journey on finding replacement calipers, another story.

In short be sure that you pressure bleed backwards from the caliper to the master cylinder and I encourage the process of putting a clear tube in the fill port running out to a catch bottle so that you can see good backflow and removal of any bubbles. Use plenty of hydraulic flujid, don't be stingy. But realize there are parts of the system in the sidewalls that could be suspect. You may not see leaks in the sidewalls because they are hidden. And I was surprised by the micro pitting corrosion in the calipers causing pressure loss. My system still does not seem to be as "hard" as my 182 but improved from before. On my list is probably replacement of the master cylinders suspecting some wear, wallowing out of the bore, but things seem to be good enough for now.

I have attached a picture of the micro pitting in the calipers, and the connecting device from aircraft Spruce that I used to pressurize and back bleed the system (connected to a garden sprayer to make a home pressure bleeder; you can buy one from Spruce with all the parts).

http://www.aircraftspruce.com/catalo...es/ats1246.php

http://www.337skymaster.com/messages...1&d=1649733211

[Kim Geyer]

Parking brake valve could be swapping fluid from side to side. Not sure which valve the D model has but I had to rebuild the valves on 3 of our T337H’s

[n86121]

Going into Baltimore on AM, their GA runway was closed, so I had to mix it up with back to back 737's.

ATC had me doing 360's to the side of the morning rumba line into BWI waiting for a spot to break in.

The controller said, "86121, when you come out of your 360 I need you right on the tail of the 737 at your 2 o'clock. Can you maintain 120 to the runway, there will be another 737 behind you?"

I said, 86121, roger"

I always touch the brakes before takeoff, so knew they were good.

What I then learned was when you touch down at 120 mph and touch the brakes....
formerly incompressible water in the brake line instantly becomes steam,
and you lose your brake entirely.

I limped off, only able to sort of waddle to the left.

=====

Any moisture / snot will settle down toward the brake caliper.

I keep a little oil can with hydraulic fluid in tie down box.
With about 3 inch piece of PVC pipe that goes to the oil can and fits the brake fitting.

Handy to top off the hydraulic system, as well as brakes if needed.

First, using self bleeding kit, I press the brake down to blow out any snot that may have settled, which becomes visible in the PVC line.

THEN I pressure it back up with clean fluid.

[wslade2]

Moisture settling must have been what caused the pitting in the walls of my calipers.

may be good idea to intermittently bleed the brakes at some interval besides hardly ever.