Mushy brakes

YankeeClipper · #1 12-22-23, 06:14 PM

My IA, who has been working with at least two other skymasters, has been struggling with my brakes. Every time he tries to bleed them he struggles to get good pressure at the pedals. The Masters have had all replacement parts replaced, the swivels have been rebuilt, and no fluid is leaking out anywhere. Has anybody run into this, or does anybody know of a secret to refilling and bleeding the brake system? I think all units are the same, but mine is a delta model if it matters.

n86121 · #2 12-23-23, 10:31 AM

The cylinders by your feet are 'self bleeding,' as their teeny weeny 'plugs' have a small hole.

Fill from the bottom up, at the wheel, until it drools out on the floor inside (put down rag!)

If there is air in the line, keep going until there isn't.

Or drain the whole line, then start clean from the bottom up.

Your goal is clean brake drool coming out the top of the cylinders by your feet.

--

Once topped my brakes get hard, but not 'rock' HARD.

Meaning from hard braking to no more compression maybe 1/4 inch pedal movement.

YankeeClipper · #3 12-23-23, 10:36 AM

That's pretty much exactly the process that my IA has been describing. Frustrating. He has repacked both masters and ensured that there was no visible scarring inside. Just recently, as some of you may have read, I replaced the swivels where there was a tiny amount of weepage. We're not seeing brake fluid anywhere else at all, and yet he keeps getting mush.

Joes · #4 12-23-23, 07:27 PM

I add one step, that is to pull a vacuum on the brake line system from the bleeders. I have a Snap On vacuum brake bleeder #BB9018 and use a large oil can oiler for the push upward from the bleeders. Hope this helps, Joe S

Dan schultz · #5 12-23-23, 09:29 PM

I recently bleed mine very troublesome, eventually used my pressure pot from the calipers, installed a fitting where the vent plug is on the master cylinder ran a hose from it to a can, quite a bit of fluid run through it until the air was removed, PITA.

Dan

wslade2 · #6 12-24-23, 07:34 PM

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 · #7 12-24-23, 10:25 PM

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 · #8 12-25-23, 03:15 PM

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 · #9 12-25-23, 10:39 PM

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.

Rick Erwin · #10 12-27-23, 02:34 PM

Does anyone have any idea as to what the normal brake pressure should be when fully applying pressure to the pedals?

YankeeClipper · #11 12-28-23, 08:37 PM

Based on some of the helpful feedback you all have been giving, my IA has decided to go ahead and replace at least one of the brake cylinders. I'm not sure this is really where I'm taking on air but they are fairly old and it's worth a shot. They were of course repacked with new bits recently, but he wonders if, as one of you have pointed out, it's not leaking out past the seals.

So in my search for this I've noticed that Cessna part numbers are different from left hand and right hand, yet the Cleveland part number for my Delta model shows only one part number: 30 - 61D. Then, confusing matters all the more for me, the Cessna part catalog doesn't really specify different brakes for different years, at least not for 65 through 71. But Cleveland does. Can I use the 30-69B that they specify for the echo models, even though mine is a delta?

RandyArizona · #12 12-29-23, 03:56 PM

Quote:

Originally Posted by YankeeClipper

So in my search for this I've noticed that Cessna part numbers are different from left hand and right hand, yet the Cleveland part number for my Delta model shows only one part number: 30 - 61D. Then, confusing matters all the more for me, the Cessna part catalog doesn't really specify different brakes for different years, at least not for 65 through 71. But Cleveland does. Can I use the 30-69B that they specify for the echo models, even though mine is a delta?

Ok, you need to slow down a little.

Do you have cleveland brakes?
Has your IA checked, and / or replaced ALL of the hoses? The big one, that is always overlooked is the short one, from the hard line to the caliper.
It has a hard outer shell, but it can leak, or suck air as it hardens, and gets old.
S2178-4-0057A Fig 46 item 23
(I think it crosses to another number)
It takes alot of abuse being where it is.

I changed out my calipers a few years back, and I think I got the 30-69B with pins, and of course the AN fittings.
The calipers are "one size", and for left or right, you just put the AN fittings on the "up or down" side. The pins are are just driven in, with a mallot and a block of wood, (measure for the depth)

As for the different numbers, (30-61D, or the 30-69B), I am going to have to do more research.
Regards,
Randy

RandyArizona · #13 12-29-23, 04:07 PM

Cleveland has a component mx manual, AWBCMM0001
Section 101 has a troubleshooting table.

Get it here. https://www.parker.com/literature/Ai...AWBCMM0001.pdf

It is cleveland specific, but good info for all brake systems.

I am still looking for specific numbers, if you do NOT have cleveland brakes.

If you do NOT, please send your aircraft S/N so I can look it up specifically.
Regards,
Randy

RandyArizona · #14 12-29-23, 04:10 PM

Quote:

Originally Posted by Rick Erwin

Does anyone have any idea as to what the normal brake pressure should be when fully applying pressure to the pedals?

Rick,
Hope your "Peak" was good.
The normal brake pressure number is listed somewhere, on some Cessna Geeks desk......... As for what it is, I guess my answer would be, "enough to stop the plane".
I'm sure someone could figure it out, , hose length, hose ID, master cyl., diameter, and length of throw, pressure to master cyl piston.... etc. etc...
You get the point...
Have a good new year, up coming.
Regards,
Randy

RandyArizona · #15 12-29-23, 04:19 PM

Here is the cleveland wheel and brake cross page for the early D model aircraft.
(everyone else can insert their model, and get the data.)

https://www.aircraftwheelandbrake.co...+thru+337-1193

It shows 30-61D
With exploded view, the AMM which I referenced earlier, and a Tech service guide manual.

Good luck.
Regards,
Randy