Leaking master cylinder

jcraver · Post by **jcraver** » Tue Nov 04, 2003 4:32 pm

I have brake fluid leaking out around the piston rods at the top of the master cylinders. Looks to me like I should replace the o-ring (AN380-2-2) and the Lock O Seal (pn# 200AC-6)* I have not been able to locate the Lock O Seal at any of my usual sources. 1954 170B. Any Ideas?
jc
*I don't have the book in front of me, but the part was called Lock O Seal or something similar.

N170BP · Post by **N170BP** » Tue Nov 04, 2003 7:03 pm

Yep, 200AC-6 Lock-O-Seal - Piston Rod. I believe that's a Parker/
Hannifin part number (?). I got mine at Spanaflight (in Puyallup / Thun
Field, Washington State) See: http://www.spanaflight.com/

Also see:

http://www.sacskyranch.com/acatalog/Sac ... er_57.html

Maybe call the guys at Sacramento Sky Ranch to make sure that kit will
work on the 170 (we rebuilt a set of master cylinders for an early 180 and
they looked identical to me....).

While you're there, make sure you have the little vent holes drilled
in the reservoir plugs/caps per the service bulletin that's out there
(can't put my hands on it at the moment).

Bela P. Havasreti
'54 C-170B N170BP

N170BP · Post by **N170BP** » Tue Nov 04, 2003 7:53 pm

I looked, and the hole drilling procedure for the caps/plugs
is in the 100 series service manual, not on a service bulletin.
Sorry for the error....

Bela P. Havasreti
'54 C-170B N170BP

N73087 · Post by **N73087** » Wed Nov 05, 2003 10:14 am

The system is designed to have enough fluid in the reservoir to compensate for wear of the pads.
If the pads were worn down some, and the reservoir was topped off, then there is too much fluid in the system. If you then replace the pads, then the excess fluid will go the only place it can, out the top.
The O-rings and Lock-O-Seal will affect braking action, but I don't see how they would cause fluid to come out the top.
Dave

jcraver · Post by **jcraver** » Wed Nov 05, 2003 4:35 pm

Thanks! Spanaflight had them. The part # was changed to 800-001-6. This is only the second time I have delt with Spanaflight for parts. Those guys are the the most customer friendly and helpful people I have dealt with. All for $4.00 worth of parts.
jc

jcraver · Post by **jcraver** » Wed Nov 05, 2003 6:59 pm

N73087 wrote:The system is designed to have enough fluid in the reservoir to compensate for wear of the pads.
If the pads were worn down some, and the reservoir was topped off, then there is too much fluid in the system. If you then replace the pads, then the excess fluid will go the only place it can, out the top.
The O-rings and Lock-O-Seal will affect braking action, but I don't see how they would cause fluid to come out the top.
Dave

Looking at the parts diagram, I see your point, but I lost about 1/3 of the fluid in the cylinder out the top in just a couple of days. It leaks out around the rod when the brakes are applied. Would that be normal? Brake pads are about 1/2 way. I just replaced the break lines from the fuselage down and bled the brakes, filling to just below the top of the cylinder. I't's a new plane for me so I have no history on the leak.
jc

N170BP · Post by **N170BP** » Wed Nov 05, 2003 7:46 pm

Sorry for repeating myself, but is the vent hole there
in the master cylinder cap/plug?

I don't have my 100 series service manual with me, but
it says if a hole isn't there, to drill one (small dia.) right
through the center of the cap.

The master cylinder reservoir is just that... a place for
the brake piston to draw fluid from. If fluid is coming out of
the top, it's most likely over-filled (especially if it wasn't
"leaking" prior to your replacing the brake lines and
back-bleeding the system).

Bela P. Havasreti
'54 C-170B N170BP

Post by **GAHorn** » Wed Nov 05, 2003 8:20 pm

I like the SkyRanch folks, and doing business with them is always good because they are so informative. Their price on this item is $3.21.
At cessnaparts.com (Hill Aircraft, a genuine Cessna distributor) it's $1.93.

Also, when looking for brake parts, beware that McCauley wheels and brakes parts (frequently found on Cessnas) are not the same as Clevelands. Be certain of what you're seeing.

jcraver · Post by **jcraver** » Wed Nov 05, 2003 8:30 pm

Bella,
Yes, both have holes and the holes are clear. You can see the fluid seeping out when you press on the brake.
jc

Post by **GAHorn** » Thu Nov 06, 2003 12:53 am

They certainly sound as if they are overserviced with fluid.

zero.one.victor · Post by **zero.one.victor** » Mon Dec 01, 2003 4:31 am

What's the purpose of the vent holes in the cap?
I just put new brake pads on my double-puck Clevelands (forty-four bucks worth,thank you very much!). I then serviced the master cylinders,they both took some fluid--the LH much more than the RH. The RH cylinder had a metal plug (straight thread,NOT a tapered pipe thread) with a very very small vent hole . It worked fine. The LH cylinder had a plastic plug (tapered pipe thread) with a somewhat bigger vent hole. When I released the brake pedal after pushing it down,fluid burped out of the vent hole. This happened repeatedly,now I can see why the LH cylinder took way more fluid. The plastic tapered plug fits in the RH cylinder hole but never bottoms out,it just keeps threading in there. The straight metal plug doesn't seem to want to start in the LH cylinder.
I would like to replace both plugs. I see in my 1948 IPC that there are both supposed to be p/n 9510292 "plug-pipe". Anybody have an AN or MS number,or better yet,a physical description (for example,"an 1/8" pipe plug")?

Eric

Dave Clark · Post by **Dave Clark** » Mon Dec 01, 2003 2:26 pm

Eric. All that I've had experience with are 1/8 pipe plugs. Don't know the size of the hole. The straight threads and the pipe threads are pretty close to the same in that size so I suppose someone could have forced the straight thread one in the top or maybe even ran a tap in it. I wouldn't worry about it as long as it stays in.

N170BP · Post by **N170BP** » Mon Dec 01, 2003 5:47 pm

I'd have to dig the service manual out again and re-read the page
in question (that suggests you drill the vent holes if they're
not already there).

Memory says that without the vent holes, brake fluid can get
pumped past the piston rod/hole at the top of the master
cylinder. If fluid is coming out of the top of the vent hole(s),
the reservoir has likely been over-filled.

By nature of the design of the master cylinders, the reservoir
plugs are not supposed to "contain" any sort of pressure. They're
just there to provide a convenient way to add brake fluid and also
to keep dust/dirt out.

Bela P. Havasreti
'54 C-170B N170BP

Post by **GAHorn** » Mon Dec 01, 2003 8:42 pm

As the brake linings wear, the caliper piston extends further out of the wheel cylinder. That's why disc brakes never need adjustment...because the pistons never retract after brake applications. They only apply pressure to the linings or they don't depending upon whether or not your foot is pressing on the master cylinder actuating rod.
As the piston follows the linings and extends further out of the wheel-cylinder, the wheel-cylinder is refilled with/accepts more fluid. That fluid comes from the master cylinder and must be replaced by air (just like your fuel tanks.) That's why the fluid re-service plug must be vented.
The master cylinders have sufficient fluid capacity and quantity to provide enough fluid for the life of the linings. Replacing the linings, and pushing the wheel-cylinder piston back into the calper pushes the excess fluid back up the line and back into the master cylinder. The life-cycle of the linings begins again.
But if at mid-life the master cylinders are re-filled with fluid, and then subsequent to that action the brake linings are replaced and the wheel-cyl-piston is pushed back into the caliper....the excessive fluid that was added may be ejected out the top of the master cylinder through the service plug. Or, if the master cylinder is absolutely FULL of fluid, normal brake-pedal action will splash fluid out of the plug. The answer is to wipe it up and quit adding fluid at the brake-lining's mid-life. Remember, the master cylinders don't have to be absolutely full in order to perform their job of having sufficient fluid to service the brakes. (I suggest you service the master cylinders to 3/4" down from the top with new linings and leave things alone unless/until a leak shows up. The brakes will get "soft" and/or the pedal will develop excessively long travel before the brakes are applied--- is a symptom of low fluid.)

zero.one.victor · Post by **zero.one.victor** » Tue Dec 02, 2003 12:19 am

I "seviced" the master cylinders with 5606 until they were FULL,up to the plug hole. I guess the LH cylinder will quit burping fluid when it gets to a level that it likes. I'm kinda curious why the plug in the RH cylinder didn't burp fluid out the vent hole,I filled (overfilled) them both equally.

Eric