Left Brake Mush, can't find the culpret

[Boilermonkey]

On our M20M Bravo we have replaced the seals on the calipers and master cylinder (twice).   The left brake still gets mushy after about three flights and air is in the line.   It is only happening on the left side and we do not use the parking brake.   No signs of fluid anywhere.

Any ideas of where air might be getting in?

[A64Pilot]

Inspect to see if for some reason the pads might be getting pushed back, like loose or misaligned caliper, or more likely corrosion on the pins keeping the caliper from freely sliding or a warped disk, although a warped disk should give a pulsating pedal, but check while your there. Pushed back pads give a mushy pedal like air does.

Only time you need to change O-rings is if they leak fluid, I’ve never seen air leak in, there should never be a vacuum, any tiny suction should pull fluid from the tank, in fact that line is under a tiny bit of pressure as the tank is the highest point in the system.

Inspect the supply side plastic tube from the tank to the cylinders, if in fact air is getting in I’d suspect it would have to come from the suction side not the pressure side.

But I think it’s unlikely. Check to ensure the tank and line aren’t blocked, and the tank is full.

Finally once your darn sure everything is perfect and you have checked by removing the caliper and cleaning the pins, I’d swap master cylinders from side to side, but as that’s a bunch of work I would do it last.

I suspect you have either trapped air, or a caliper that’s stuck, I lean towards a stuck caliper as that’s pretty common.

[EricJ]

3 hours ago, Boilermonkey said:

On our M20M Bravo we have replaced the seals on the calipers and master cylinder (twice).   The left brake still gets mushy after about three flights and air is in the line.   It is only happening on the left side and we do not use the parking brake.   No signs of fluid anywhere.

Any ideas of where air might be getting in?

Any hose or hard line joint/fitting is also a potential culprit.  

[mooniac15u]

3 hours ago, Boilermonkey said:

On our M20M Bravo we have replaced the seals on the calipers and master cylinder (twice).   The left brake still gets mushy after about three flights and air is in the line.   It is only happening on the left side and we do not use the parking brake.   No signs of fluid anywhere.

Any ideas of where air might be getting in?

Which way did you bleed the brakes?  If you bleed down it is almost impossible to get all the air out of the system.  Pressure bleeding up from the landing gear seems to work better.

[Yetti]

You may have a leaking master cylinder.   Rebuild it.   You could also just not be getting the air out.   Use a oil can and pump up from the bottom.   I (well the AP) have had to pump 50 times to get the air out.   Then year later had to bleed again.

[Mcstealth]

Check the obvious 1st. Make sure the brake pads are not in backwards.

[Fritz1]

put pressure bleeder on reservoir, need about 10 psi, trace supply line from reservoir to master for drips, when bleeding left brake pump left pilot and left copilot pedal simultaneously, hang caliper upside down with safety wire so that bleed valve is up, use old brake pads or any piece of material that has thickness of brake disc as spacer, air bubbles can be pushed aside when pilot and copilot pedals are not pushed simultaneously, air bubbles hide in top of caliper if not flipped upside down when bleeding, run liberal amount of fluid, familiarize yourself with the location of the bleed opening behind each caliper piston, this will allow you to hang the calipers perfectly while bleeding, I think we used over one qt after resealing both calipers, persistence pays off!

[PilotX]

Definitely take the belly pan off. Found a fitting under there that was letting in air once (no apparent leak though) and had the master cylinder rebuilt as it was leaking too. My left brake still get mushy on occasion. Had the shop change the oil and rebleed the left. 

[Guest]

You can cap the outlet at the master cylinders to essentially split the system in half to see if the air is in the master cylinders or wheel cylinders.

On really stubborn ones you can crack the line at the union one the wheel well to bleed air, it’s about the highest point in the system.  Or install Tee instead of the union and then install a cap on the Tee as a bleeder port.

[Boilermonkey]

Always great advice on MS.  Thanks folks, I've got my to-do list for the weekend!