Copilot brake bleeding woes and solution

[spitzfyre]

Hello, I am posting this in the hopes that I can save someone major headaches and frustration in the future when it comes to bleeding the brakes; specifically, the copilot side brakes. From what I understand, not many of our birds have these installed, and mine happens to be one of the rarer ones that does. I've spend 8+ hours in the past few days trying to get it right so here is my writeup so hopefully it may help someone in the future.

So, annual time on my Mooney. I removed the leaking parking brake for cleanup and rebuild, as well as replace one of the brake lines. Before removing these components, I decided to simply open the bleeders at the calipers to let the system completely drain. Figured I'd give it all new, fresh fluid while I was at it. After completing my work I bled the brakes as per normal procedure, ie pushing fluid from the caliper up through the system to the reservoir. My friend reported no more bubbles, cool, switch sides and complete procedure again. Afterwards I climb into the cabin to test the brakes. Pilot side feels great while copilot side goes straight to the floor. 

I won't bore you with all the details of the troubleshooting but it was several head-scratching hours of frustration. Rebuilt the copilot master cylinders thinking that maybe they were letting in a small amount of air (I got this idea from someone else's past post here on Mooneyspace), which did not fix it. Eventually I removed the belly panel I'd previously installed, thinking I was nearly done with my annual and didn't need access above it anymore, and traced the brake lines back to find two shuttle valves that I had previously not known existed. Apparently, part of the installation of the copilot brakes is to put these shuttle valves in; with these valves, whichever set of brakes is depressed will push the valve over and allow fluid to go to the calipers. This, of course, means that both sets of brakes cannot be used at the same time, which really doesn't make much sense to me. Maybe its purpose is to protect the calipers from too much pressure from both pedals depressed at the same time...? I don't know. But anyway... I removed the shuttle valves and made sure they were not seized/frozen. Replaced the O-rings while I had them out. Reinstalled.

So here's when all the pieces came together on why the pilot brakes were bleeding fine but copilot's were not; in order for the shuttle valve to move, the pressure has to come from the pedals to push it over. Pushing fluid up from the caliper is putting pressure on the wrong side, so the fluid all goes to whatever side the shuttle valve happens to have open at the time; in my case, the pilot side. So we bled the pilot side like normal, then I took an air nozzle and shot a quick blast of air into the shuttle valve where the line to the copilot brakes should be connected. This air pushed the shuttle valve over. So then I reconnected that line and we bled the brake again, this time the fluid was directed to the copilot side. Repeat entire procedure on the other caliper.

Now the brakes all work great.

In my searches for solutions to copilot brake bleeding problems on Mooneys, I was not able to find anything that helped, so I really hope that someone who needs it finds this writeup someday.

[DonMuncy]

Thanks for the report. I'm sure someone will benefit from it.

[MooneyMitch]

Yes sir!  Same issue with my C/D model pilot/co-pilot brakes as well a few years ago.  I learned about shuttle valves too.  Leaking O ring in the valve.  Replaced all O rings in the valve assemble...................all was good.

Thank you for sharing your detailed description. 

[cliffy]

Yup well known problem bleeding dual brakes.

Our local shop spent 3 days on one last summer. 

I'll pass alone the good idea of blowing the shuttle over to them

Thanks!

[Yetti]

Not sure about using compressed air.   Would a brake pump booster be enough to move it.    The fear with air would be introducing more air into the systems to get out.     We have done 40 pumps and then another 40 pumps with an oil can from the bottom to get all the air out.

[spitzfyre]

26 minutes ago, Yetti said:

Not sure about using compressed air.   Would a brake pump booster be enough to move it.    The fear with air would be introducing more air into the systems to get out.     We have done 40 pumps and then another 40 pumps with an oil can from the bottom to get all the air out.

It shouldn't introduce any extra air because you are blowing into the side you haven't bled yet and closing off the side you did bleed. After pushing it over, you can connect the line that goes up to the brake pedals and then push fluid from the caliper up, thus bleeding all the air.