Brake master cylinder

[gfeher]

I've always found bleeding from the bottom to be much more reliable on light aircraft brakes than bleeding from the top. You really don't need to push much fluid up the lines as any air tends to want to go up. I recently bled a brake line (from the bottom) after the flexible line at the caliper was replaced and I gave it a couple of pumps after the fluid came out of the top of the master cylinder and the lines were clear. Since your master cylinder is empty, your lines should be clear by the time the cylinder is full and fluid is coming out of the bleeder hole.

[c170b53]

Doug, sometimes I find it difficult to use english effectively to convey a thought about a mechanical device. Its also difficult to know when giving advice to know the effective detail level needed for the recipient, especially if you do not know the receiver. So I apologize if I’ve gone over the top. I like when owners get involved, especially if they take the time to progressively learn, develop skill and learn patience.
So good for you for not throwing up you hands.
I think you should be able operate and gauge the valve operation with all the parts out of the cylinder on the bench. Like Gene, I like the bottom up method, you do not need a partner, place a rag around the master cylinder and pump away. Maybe its the time to orientate your bleeders to the bottom. Or remove your caliper (if you use a flex line from a solid line on your gear) turn it upside down, clamp the piston so it doesn’t inadvertently pop out and try eliminating the air that way. Remember choice words help!

[dstates]

Jim,

I appreciate your help and ideas. I am trying to become as familiar as possible with my plane and my A&P is great about letting me do a lot of the hands on stuff. I have learned a large amount already and continue to do so.

I like the idea of taking the caliper off and letting it hang upside down to help bleed. I hope to have this sorted out by end of day tomorrow

Doug

[n2582d]

Doug,
I'd have to agree with John in that the problem is the bleeder needs to be on the bottom if you want to clear the air out of the caliper. Just be sure you don't dawdle as you diddle with that deal or the 5606 will puddle as you piddle with the pedal.

One thing I was curious about is that you show two nuts in your photo. The IPC fig. 59-19 shows one AN365-632C all metal stop nut. That's an obsolete number as is 22FH632 which later C-172 IPCs illustrate. While this has nothing to do with your current bleeding problems I thought this old General Aviation Airworthiness Alert gave some good advice regarding replacing these nuts:

Click to Enlarge

[dstates]

Doug,
I'd have to agree with John in that the problem is the bleeder needs to be on the bottom if you want to clear the air out of the caliper. Just be sure you don't dawdle as you diddle with that deal or the 5606 will puddle as you piddle with the pedal.

One thing I was curious about is that you show two nuts in your photo. The IPC fig. 59-19 shows one AN365-632C all metal stop nut. That's an obsolete number as is 22FH632 which later C-172 IPCs illustrate. While this has nothing to do with your current bleeding problems I thought this old General Aviation Airworthiness Alert gave some good advice regarding replacing these nuts:

Screen Shot 2020-04-24 at 4.28.38 AM.png

Gary,

I had noticed the nut difference too, but didn't give it much thought. I appreciate you sharing that information!

Doug

[johneeb]

Doug,...……….. Or remove your caliper (if you use a flex line from a solid line on your gear) turn it upside down, clamp the piston so it doesn’t inadvertently pop out and try eliminating the air that way. Remember choice words help!

Gad Jim this has to be why you are paid the big bucks, such a clever simple way to deal with bleeders on the top of the caliper.

[gfeher]

Doug, I forgot that you mentioned that the bleeder is at the top of your caliper. I agree with Jim that if you have flexible lines at the calipers, you should slide the caliper off the posts and orient it as best as possible so the bleeder is the lowest point in the system. And as Jim mentioned, don't forget to clamp the piston first!

[c170b53]

John, getting big bucks along with the mentioning of possible pension bridging are the two standard giggles for those in aviation that never get old, thanks for that.

[ghostflyer]

In our workshop we used a plastic garden sprayer for back bleeding the brakes . It held about 1.3 gallons of the raspberry red [H5606] and had a shut off valve on the plastic pipe that attached to the bleeder . Worked great . We had a young mechanic who was bleeding the brakes on a Cessna 185 that had new seals on the pistons and new pads . He was having difficultly on getting the flow to the master cylinder. He wasn’t happy by the flow of words. “ the bleeder is open ,the shutoff valve is open %%$&$. Some one yelled it’s nearly lunch time have a break. We were all having lunch in the lunch room when a customer came walking in looking for somebody remarked there a heap of red fluid running out of a aircraft . Yep, red oil was running down the bottom of the fuselage towards the tail wheel and spread very nicely across the floor . The young mechanic had mixed up the position of the shutoff valve on the brake bleeder[garden sprayer] . He had opened it when he went to lunch thinking he had switched it off. Raspberry [new name for the young mechanic] had to remove seats , floor panels and clean the inside of the fuselage and the hangar floor. He finished about 10pm .
PS. That hangar floor was sticky for months .

[ghostflyer]

Just talking to a friend about the brake Fluid everywhere and having a few laughs and he tells me the young mechanic is called “raspberry “ by his WIFE.

[GAHorn]

Art Linkletter to child: “So, your Mother and Father met in a bakery! Does your Mommy call him “cookie”?

Child: “No... she calls him “crumb”.

[Bruce Fenstermacher]

I'm just getting back to this thread and wanted to tell Doug he is correct and my observation of the drawing was not. It is indeed .040" from the washer of the lock-o-seal to the piston. When I was looking at this (on my phone) I was thinking a o-ring, which you can plainly see in the drawing, forgetting I was looking at a lock-o-seal which has a washer around the seal. I've added a correction in read to my original post.

The rest of my comments still stand. the piston and the lock-o-seal must seal when the piston rod is pushed down or no fluid with flow, And the seal must be broken when the piston rod is moved out or the rod is not likely to return tot the top or at least as quickly as need be. This can easily be tested on the bench.

It sounds like Doug may have air in the system due to bleeders at the top of the calipers and bleeding from the bottom up if I follow that correctly.

War Stories. I think if you've bleed more than a system or two, we all have them. Mention bleeding the brakes on a Cherokee to a mechanic and you are sure to hear a few. The Cherokee, as most are equipped today, has 5 master cylinders. One is the parking brake mounted horizontally, the other 4 are each mounted at the pedals vertically but upside down. Nearly impossible to pressure bleed from the bottom while pumping the master cylinders as the manual suggests. The manual also suggest you might have to disconnect the master cylinders at the bottom pedal pin and rotate them right side up best you can to remove air. Not the easiest task with the rats nest of hydraulic hoses still attached. Many have found that once you get the parking break filled with fluid, by pumping it you can bleed top down while cracking each B nut in line from the pump to the end of the master cylinders. In most cases it takes all the above and it is impossible to catch fluid from a loosened B nut. Lots of fun. This is what I was doing last week.

In contrast the 170 has a simple system.

If you haven't already Doug, you will get this.

[dstates]

Thanks everyone for the help. Yesterday I double checked the adjustment of the master cylinder lock-o-seal and the cylinder worked great on the bench. After some frustration getting the cotter pin in the bottom clevis pin and bent correctly, we were able to bleed the cylinder. We had a lot of bubbles come out of the vent port on top of the master cylinder. After a while I had a nice working brake. Good pressure. So we said, let’s leave it sit and wait a half hour and see if anything changes like possibly more air works its way up. We should have left it alone. We bled it a little more and after that we didn’t have pressure and the master cylinder would bottom out.

Our thoughts are that either we introduced more air when we bled it again or there is something not right in the cylinder. I personally think we were introducing air. Due to poor planning we didn’t have a big jar in the cabin to catch the overflow when bleeding so we kept having to stop and empty the container we had. We are going to take a large jar next time and just run a full quart through if we have to. We also didn’t hang the caliper. That is also on the list to try.

We’ll get this.

[MoonlightVFR]

A few of us owner/pilots of C170 have been peering down into the barrel of the disassembled Master brake cylinder.
I just wanted to commend the original designer of the cylinder. It is such a unique compact design. Brilliant.

Did you notice that there is a lower portion at very bottom of cylinder that collects rubber seal worn RESIDUE. This allows the residue to NOT be pumped throughout the system. 50 + years and no problems.

I cleaned out the bottom of master cylinder with Q Tips and or cotton TAMPONS

[DoubleOught]

Having similar problems, can’t push fluid up from bottom, even after I removed and cleaned bottom bleeder. Going to get a new oil can pump tomorrow, mine is about 10 years old it might be weak. Just in case wear did y’all find master cylinder rebuild parts / kits ???