brake fluid replacement.

How to keep the Cessna 170 flying and airworthy.

Moderators: GAHorn, Karl Towle, Bruce Fenstermacher

Post Reply
User avatar
ghostflyer
Posts: 1390
Joined: Mon Apr 21, 2008 3:06 am

brake fluid replacement.

Post by ghostflyer »

Just had a interesting issue with brakes on a Cessna 206. Aircraft is 5 years old . Just had a 100hrly done with new pads . Brakes work great when cold but after a couple of go around ,the left hand brake pedal to the wall and no braking [very little] . Right hand brake had new seals fitted at the 100hrly and works perfectly . Let it cool [LH brake] and brakes are normal. I did a high speed run down the runway and applied the brakes together and next we are nearly ground looping . Pads came as a set with correct paper work. It appeared that the brake needed bleeding but when cold worked perfectly. so i decided to bleed it at the calliper . OMG the color of the oil that came out. I am sure it was a mixture of oil and water . Unit flushed out ,now all good . So when do we flush out our brake systems . Maybe every 100 hours.
User avatar
ghostflyer
Posts: 1390
Joined: Mon Apr 21, 2008 3:06 am

Re: brake fluid replacement.

Post by ghostflyer »

The LH master cylinder oil level was checked at 100 hrly and so did i. It was one of the first things I thought was the issue .
User avatar
DaveF
Posts: 1516
Joined: Sat Nov 03, 2007 1:44 am

Re: brake fluid replacement.

Post by DaveF »

Every annual I stick a small tube into the bottom of each master cylinder and draw out fluid until it’s not discolored, then top up with new. And I occasionally have to replace the caliper puck O-ring(s), which requires flushing the fluid from the calipers. So my brake fluid is always being replaced, a little at a time.
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: brake fluid replacement.

Post by GAHorn »

ghostflyer wrote:….the color of the oil that came out. I am sure it was a mixture of oil and water . Unit flushed out ,now all good . So when do we flush out our brake systems . Maybe every 100 hours.
Use of the CORRECT fluid is imperative. I’ve not personally witnessed it…but I’ve heard stories of such things…where automotive brake fluid or other fluids applicable to other systems have been used instead of the correct (H-5606) fluid.
Automotive brake fluids (such as DOT 3/DOT-4 or “Girling” fluids, etc. are usually “hydroscopic”….I.E. like alcohol they attract/absorb water and degrade.

I’m experiencing a “mystery” right now with our SkyHawk left brake. While taxying we noticed the left brake pedal was “low” and needed pumping which only confirmed it was operable for the return flight… but pumping it did not recover the pedal “height”. When we got home we could not find a leak and we serviced it, filling the master cyl with 5606.
Two flights later (a week or two passing between flights)…it re-occurred. The cylinder was again topped off. No sign of leaks anywhere.
A couple weeks later… no pedal again.

We pulled the master cylinder, resealed it, new Lock-O-seal, everything inspected, reinstalled and new caliper seal and brake linings. A week later…low pedal… requires “pumping” to get a firm pedal.

8O 8O
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
User avatar
Richgj3
Posts: 189
Joined: Wed May 27, 2020 3:13 pm

Re: brake fluid replacement.

Post by Richgj3 »

This is a long shot, but in the older Corvette world similar things have happened caused by the disk not being true. The oscillation causes tiny air bubbles in the brake fluid. Just a thought.
Rich Giannotti CFI-A. CFI-I SE.
1952 C170B
N2444D s/n 20596
User avatar
ghostflyer
Posts: 1390
Joined: Mon Apr 21, 2008 3:06 am

Re: brake fluid replacement.

Post by ghostflyer »

some leaks are not that apparent ,but i would run my hand down the under side of the fuselarge from the gear leg to horizontal stab. If it’s sticky ,the pipe flange where the fuselarge fitting that joins on the brake line that runs down the undercarriage leg could have a small crack in it. Other wise i would be looking under the floor for leaks .
Another gotcher is where the brake pads have worn down so far there is small leak from the pistons in the brake calliper when brakes are applied .
User avatar
Desertflyer
Posts: 2
Joined: Mon Dec 19, 2022 3:09 am

Re: brake fluid replacement.

Post by Desertflyer »

I had a similar leak of the fluid from one side of the brake system and could never find a leak. I bought the plane and the brakes had no leaks , no low fluid for years. Then I discovered that the black screw-in plugs for the master cylinders were laying next to the master cylinders. It was amazing they hadn't rolled all over and got lost in all the flying. So of course I put them in. Can't have open master cylinders. Then I had the fluid disappearing from my left master cylinder. No leaks could be ever be found. And I REALLY looked. Then I learned that the screw-in plugs for a 180 are drilled for venting. AHA! Yep there it was. The left master cylinder plug wasn't drilled and whenever the brake was released fluid would travel up around the vertical actuating rod and between the rod and the cap and spread out and come down over the top of the master cylinder and into the bottom of the plane and just disappear. I could get my head in there and actuate the brake and just barely see any come up next to the rod but it did, but only when you RELEASED the brake. I drilled the hole in the plug and that was the end of the plroblem.
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: brake fluid replacement.

Post by GAHorn »

Follow-up of my earlier post: Problem solved.

There was no leak of fluid. This airplane had flexible lines just above the caliper which were too-long…and “looped” slightly above the landing-gear-spring before attaching to the aluminum/metal line following up the landing gear spring. As the hoses “aged” they have become stiff…losing flexibility…. resulting in the caliper being “twisted” slightly-away from the bracket which holds the pins of the caliper.
This results in the caliper moving away from the disc when brake pressure is released….slightly expanding the caliper…in other words, causes the brake linings to move away from the disc…opening-up a space instead of the linings remaining in close-contact with the disc…. Which results in the need to “pump” the brake pedal to take up the “slack” and move the linings back into close-contact with the disc.

Replacing the hose at the caliper using the proper-length, avoiding excess length, resolved the issue. Now the caliper does not move away from the disc when brake pressure is relaxed.

(Also, the 45-degree fitting on top of the caliper was not aligned fore/aft but instead aimed toward the cockpit slightly…which contributed to the side-ways “twisting” of the hose/caliper forces. Re-aligning the fitting exactly fore/aft eliminated that contribution to the issue.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
User avatar
mmcmillan2
Posts: 161
Joined: Sat Jul 04, 2020 9:30 pm

Re: brake fluid replacement.

Post by mmcmillan2 »

Are these hoses custom length or is there a part number? Mine are probably 15+ years old. Still seem flexible but I may get new ones next annual.

Yesterday we put new o rings and fluid in my 170. Right brake had gotten weak, but now both feel awesome!
170B owner, KCFD, CFI(I), ATP Multi
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: brake fluid replacement.

Post by GAHorn »

Typically it’s best to make-up the hoses custom …but if you are happy with the length of existing…you can measure from mating-surface to matich-surface of the fittings and order from Aircraft Spruce…or Precision Hose. I make up my own as I own mandrels and had the mat’ls.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
Post Reply