Brakes,I need a break

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

Brakes,I need a break

Post by ghostflyer »

Due to floods ,my undercarriage was covered in water and sewage . I had to repair/replace my rims,bearings and brake callipers. I couldn’t purchase a new twin spot calliper so it had to be repaired . Repaired and tested . The Cessna 170 brake system is a single line very simple brake system. So I thought ???. Reservoir was filled and and brakes pumped with the bleed screw actioned as required . Finally we were getting a clear stream of oil from the bleeder. Brake pedal was still going to the “floor” with no resistance . Reservoir was checked full. Ok we will back bleed it from the calliper . Can’t, a hydraulic lock . Pedal wouldn’t move ,locked solid. Ok we have a problem with the actuator . It was removed and checked with the “lock-O-seal gap checked at 40 thou. All new seals fitted . The floating piston face surfaced finished as it had some marks on its face. The spring under the piston looked a little distorted . Then shown a old one that somebody had in a box of brake parts . Mine looked better condition. Actuator body removed from pedals [isn’t that a fun job]. Bore of actuator checked/measured,all in very good condition. So after 46 man hours later of checking this and that etc , this is a “simple” brake system isn’t it?? I was trying to back bleed it from the calliper AGAIN, and a hydraulic lock was occurring . Sorry need a break ,will continue later.
User avatar
ghostflyer
Posts: 1390
Joined: Mon Apr 21, 2008 3:06 am

Re: Brakes,I need a break

Post by ghostflyer »

Well after pulling the aircraft pedal area apart ,checking all the brake lines for kinks and security etc now trying to back bleed it from the calliper ,I notice some very tiny bubbles coming from a B nut on the brake line that connects to the right angle fitting that goes through the fuselarge . It wasn’t very tight at all . I tighten it up and then the hydraulic lock disappeared and oil started to flow into the reservoir . . Bleed screw locked off, brakes normal. So what caused the hydraulic lock…? I had the pressure pot to high in pressure ,it was over 120 psi . “some body had by passed the pressure regulator on the compressor . Normally the air supply that I use is around 60 psi. The pressure pot was slowly bleeding down due to the pipe leak . I didn’t see the oil spill from the B nut due to a card box full of rubbish under the aircraft . I wasn’t expecting a leak ,I was only focused on the components that were changed . Oil loss for the entire brake bleed was very little . Wife was “helping”??? but we will not go there . Time to bleed L/H brake system about 10 mins ,time to bleed R/H system about a week. The pressure pot is about 10ins dia and is spherical and about 1/2 full so what’s the head of pressure there?.
Yes I checked with a number of “experts” and they had a series of theories and advise [including burn the bloody thing] but now it’s time for a break. :D
User avatar
gobrien
Posts: 113
Joined: Sun May 19, 2019 11:36 am

Re: Brakes,I need a break

Post by gobrien »

On the other side of the world, I was struggling with a similar situation on Saturday. I too took a break!

This may be well known, but it was new knowledge to me and might help someone. I had read about pressure filling the system from the bottom, but it was not working for me. I could pump in a small amount of fluid but then the pressure would just build and no more would go in.

Yesterday (Monday) I went back at it and one of the lads in the hangar helped out. I had expected it would back fill and bleed from the bleed nipple on the caliper with the pedal in the up position, hence the struggle on Saturday. There followed an hour of messing, puddles of hydraulic fluid caught in a bowl and mopped up with paper towels, plus a lovely blow out with me splattered with the stuff (not tasty). What ended up working was to pump fluid in through the bleed nipple with 1 bar of pressure. With the pressure holding and about 100ml in the pump, my copilot pressed the brake pedal all the way down and then slowly let it back up. As the pedal came up the fluid went in via the bleed nipple and the air bubbled up through the resevoir. We repeated this a couple of pumps until the bubbles stopped, removed the pressure at the bleed nipple and closed it. L brake pedal is good.

Got the R pedal to the same state and then discovered a leak in the newly replaced hose fitting. So new fitting ordered to remake that.
1948 170 Project (N4180V) now EI-AEN SN:18513 - Dublin, Ireland
https://www.taildragger.eu/
User avatar
daedaluscan
Posts: 497
Joined: Fri Apr 20, 2012 7:03 pm

Re: Brakes,I need a break

Post by daedaluscan »

When I upgraded to double puck ABI brakes I took my master cylinders out and rebuilt them. Not difficult at all and a satisfying amount of goob was sitting around the base of the cylinders. After adjusting the valve to specs they bled easily from the bottom with an oil can full of hydraulic fluid. I opted not to try to catch the fluid as I had the interior out and just did a good clean up. That, combined with bushing the worn out rudder pedals, gave me a much more positive and confidence inspiring feel. I love the extra braking power.

I think if the cylinder is correctly set up they should bleed fine from the bottom.
Charlie

1956 170B C-GDRG #27019
User avatar
Bruce Fenstermacher
Posts: 10313
Joined: Tue Apr 23, 2002 11:24 am

Re: Brakes,I need a break

Post by Bruce Fenstermacher »

I can not think of a reason a properly operating and adjusted master cylinder would not bleed from the bottom. Think about it. If pressure can not be relieved by fluid flowing back into the master cylinder, the brake cylinders would not be released in operation.

The one way valve in the master cylinder may not letting fluid back in if the pedal and plunger rod is not fully retracted when at rest. And a properly operating master cylinder should not suck fluid back into the reservoir as the pedal and plunger is retracted either.

BTW if your system has a valve type parking brake like those found on later aircraft, you can't pressure bleed the brakes from the bottom with the parking brake on. Took me 2 hours once and and new pressure pump (garden sprayer) to figure that out.
CAUTION - My forum posts may be worth what you paid for them!

Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
hilltop170
Posts: 3481
Joined: Sat May 06, 2006 6:05 pm

Re: Brakes,I need a break

Post by hilltop170 »

Try disconnecting the brake plunger from the pedals, it’s just a cotter pin removal and slide the rod out of the clevis. Then try to pump fluid up from the bottom of the brake caliper bleeder. Open the bleeder 1/2-1 turn then pump. If everything is right in the master cylinders, it should pump thru freely. If it doesn’t remove the master cylinders and rebuild them with correct parts. While you’re there, inspect the rudder pedal pivot holes for excessive wear.

This would be a good time to remove the pistons, clean the pistons, caliper bores, and guide pins, and replace the o-rings. Use Parker-O-Lube (from Spruce) to lubricate the new o-rings and piston bores prior to reassembly. Also, spray dry graphite lube (from the auto supply) onto the guide pins. Obviously, replace the brake shoes if they need it.

Put a 1/8npt x hose barb plastic fitting (from the auto supply) in the master cylinder and catch the fluid in a bottle. I always drain out all the fluid thru the bleeder and then purge at least one system volume to make sure the whole system has new fluid. Wrap a bunch of rags around the master cylinders to catch stray fluid and dropped cotter pins, yes you will!

I have also started using Royco 782 synthetic hydraulic fluid (available from Spruce) instead of 5606. 782 does not attract/absorb water like 5606 and is compatible so a flush is good enough if the master cylinders are clean (it’s always a good idea to take the tops off and clean them out anyway).
Brake bleeder pump set-up
Brake bleeder pump set-up
Brake fluid purge set-up
Brake fluid purge set-up
Richard Pulley
2014-2016 TIC170A Past President
1951 170A, N1715D, s/n 20158, O-300D
Owned from 1973 to 1984.
Bought again in 2006 after 22 years.
It's not for sale!
Post Reply