Installing Copilot Brakes Causes Pilot Brakes to Fail

[Cayman44]

Has anyone who installed the optional copilot brakes on the Mooney M20M TLS Bravo Serial 27-0021 (or other models, M20J or M20K) and had the pilot brakes become so spongy that they continuously require 3-4 pumps to bring them up to usable found  a reason why or have a suggestion.  The pilot brakes were fine before the install.  I cannot find any brake fluid leakage.  Does it make sense that both pilot master cylinders would fail at the same time. 

Even bleeding them three times bottom up makes no difference.  They firm up to only requiring 1 to 2 pumps for less than a day then go back to 3-4 even sitting.  I always find about 12 inches of air in the reservoir line behind the pilot cylinders after using them even though the bleed pushes all air to the reservoir.  I have run over a quart of brake fluid through them.  The new copilot brakes show no sponginess at all.  Next step is to pull down the cylinder belly panel itself and there is not much pulling down as it is sealed wrapping into the firewall.  I'm ready to buy new cylinders or rebuild them which ever makes more sense in the long run. But having a good feel for cause is needed and may provide a different solution.  Thanks

[jerryg324]

Check the spacers on the lower gear doors...  Sometimes they are not placed correctly and the door will press against the caliper forcing it open and resulting in a soft pedal...  Just confirm you have clearance between lower gear door and brake caliper...

[jetdriven]

Try bleeding them like a car from the copilot side.  It pulls fresh fluid into the pilot side 

[Yetti]

You have air in the lines that is getting pushed around.

[Eraaen]

Not sure if it is related AT ALL. But, I replaced brake lines on both sides.  I had to bleed the lines probably at LEAST 4 times before I got all of the air out.  Still have no idea why it should have been so difficult.

[Marauder]

Not sure if it is related AT ALL. But, I replaced brake lines on both sides.  I had to bleed the lines probably at LEAST 4 times before I got all of the air out.  Still have no idea why it should have been so difficult.

I went through the same nightmare when I replaced my lines a few months ago. Ended up buying every style of brake bleeder on the market to get it bled.


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[bradp]

I had a soft brake - Lynn found that the last time it was serviced, someone forgot to tighten 

[Cayman44]

Thanks for all the input.  So far I have not found any leaks.  Apparently the installer did not correctly bleed the brakes and actually did a test flight.  Returning home the installer told me its a bottom up bleed in the M20M.  I'm 700 miles away or would have taken it back.  He had it for two months doing quite a number of tasks for me - annual, new windshield, gear rigging because right gear door was mis-rigged from previous owner(s), etc.  The fluid goes from the brakes through the new copilot brakes and then to the pilot brakes and then out the reservoir.  I had been doing that, getting a flow through to push air out of the reservoir line that could easily be seen in the translucent line behind the pilot brakes by pulling down the front large belly panel, then locking down the bleed valve then pumping the brakes and then watching a few air bubbles come out the reservoir.  Pretty much all the fluid that I put in came out the reservoir. 

After all that I contacted the Factory and Stacy added that I should get a flow going and leave it going while slowly pumping the brakes.  I would slowly pump the co-pilot brake and noticed a very short throw to bottom out the new brakes.  Then move to the Pilot side and slowly pump and saw a really, really long throw to bottom the brake and hold it there - eventually pumping about ten times slowly.  Did that three times with the flow still going.  Each time I would go back to the reservoir and there would be no fluid coming out for a few minutes and then the flow  would start again.  The long slow bottom out of the pilot brake was pulling a lot of fluid from the reservoir into what must be massive original cylinders.  Stopping for the day I noticed that my overflow plastic simple orange juice bottle had filled to less than 1 1/2" - not much compared to my earlier tries.  Returning the remaining clean fluid in my pressure bottle back into the brake fluid bottles I bought from Aircraft Spruce, I found that I had added into the system, both brakes, about ONE pint of fluid.  Instead of being 3-4 strokes to being firm to get brakes that immediately went back again to soft after releasing, I am only one stroke away from solid firm.  So one more bleed will happen soon.  Will let you all know how it works out if I can get them truly firm - no air.  Very time consuming for  me from what I had expected and paid for.  I also learned that the original bleed nipples on the bottom of the brakes were soft metal and could be rounded and also break off.   I now have hardened ones and a spare - for you do it your-selfers.

Edited April 26, 2019 by Cayman44

[Yetti]

Sometimes flying will move some of the air around in the system

[triple8s]

I had to replace the left caliper.Although I do not have co pilot brakes, here is my story. Bled brakes every way we know, AP and I did, bottom up, top down over and over again. He said I may be getting air into the system through the brake master cylinders and we should rebuild them I thought yes he is an AP and has worked on aircraft since Vietnam War where he was a mechanic but I couldn't see how air could get in if no fluid was getting out. He pointed out the fluid level in the reservoir would go up after bleeding and that's why we weren't losing any fluid. So we removed both master cylinders and replaced the o rings in them. Bled the brakes again from the bottom up and no more problems. He said the master cylinders can pull air in around the shafts when the pedal is released. Guess he was right.

[Cayman44]

Thanks triple8s - will keep hat in mind.. This is my last post on this topic that I started unless I have to do a master cylinder re-build like above some time because of air entering, but the pilot brakes have the same feel now as before the copilot bakes were added.  I left off above with one pump of spongyness after adding a pint of fluid to the system to bring the peddles up to useable firmness.  Today I did one last bleed and added another 2-3 oz to the system.  No more pumping required !   The old original pilot cylinders really do have a long throw and are noticably softer engaging than the new copilot brakes with their short throw and really firm feel.   I have the original single puck calipers.  Cheers

[stevedietrich]

Bleeding the brakes with CP brakes takes a lot of work, patience  and some magic. 

 

Sounds like you still have some air.