Can't bleed MATCO brakes

[yankee-flyer]

During the initial installation about 15 months ago I had GREAT difficulty filling the system from the bottom up. During the Production acceptance tests I was unable to bleed the brakes from the bottom up-- I could get fluid through the caliper (opening the line at the top of the caliper showed fluid flow) but could NOT get flow out through the top of the reservoir so I bled from the top down which worked OK. I tried to bleed the brakes during the condition inspection this week and had the same problem. I called in the local A&P and he could not get fluid from the bottom up with his pressure tank. We pulled forward on the pedals, rocked them, pushed, them and tapped on the master cylinders without being able to get flow out of the reservoir. HELP!!! I'd like to bleed the brakes correctly.

Wayne Moyer RV-12 120241/143WM

 

[lewy]

Brakes

Wayne, we have same problem but we still have air in the line. We have tried bleeder in three different positions and still can't force the fluid up but we can push fluid from the top down??

 

[Dave12]

I had the same problem as many others have also. I used the pump squirter to move the air bubbles as close to the nearest fitting as possible then broke the fitting loose and pumped until the bubble disappeared. I caught the excess fluid in a container and quickly tightened the fitting. Sounds goofy, but it worked. This whole procedure is a matter of luck. A fellow 12 builder who lives up the road from me, did the procedure and blew the air right out???? Took 5 minutes.

 

[DonFromTX]

I just done mine, beginners luck I guess. I had heard the horror stories and expected the worst. I used a pump oil can and a piece of clear tubing, just kept squirting until I saw it go thru the firewall in the cockpit, gave it a few more for fill of the reservoir. Did the same on the other side, tight pedal, no air, easy as pie.

 

[DonFromTX]

That cannot be correct, and whatever is blocking could cause problems later with the brakes as well. I would just keep cracking lines from the wheel cyl up until I found the restriction, and then fix it. Some have had bad master cyl if I recall correctly.

During the initial installation about 15 months ago I had GREAT difficulty filling the system from the bottom up. During the Production acceptance tests I was unable to bleed the brakes from the bottom up-- I could get fluid through the caliper (opening the line at the top of the caliper showed fluid flow) but could NOT get flow out through the top of the reservoir so I bled from the top down which worked OK. I tried to bleed the brakes during the condition inspection this week and had the same problem. I called in the local A&P and he could not get fluid from the bottom up with his pressure tank. We pulled forward on the pedals, rocked them, pushed, them and tapped on the master cylinders without being able to get flow out of the reservoir. HELP!!! I'd like to bleed the brakes correctly.

Wayne Moyer RV-12 120241/143WM

 

[yankee-flyer]

Reply from Matco

Just got this from Matco. Is there enough variance in the way the pedals are installed to cause problems for some of us and not for others?

Wayne


There is a bypass valve in the master cylinder that opens on
approximately the last 1/16 inch of travel of the shaft. The shaft
has to be fully extended. You can verify this by disconnecting the
shaft from your toe brake assembly and extending pulling on the shaft
to be sure it is fully extended. The shafts should easily extend on
their own. If they are not smooth in their extension, there may be a
problem with them.

 

[DonFromTX]

Logically that is almost the only place a restriction would occur. I wonder if something in the assembly of the rudder pedal/brake pedal is causing so many problems. A friend burned up a wheel cylinder and disc from this problem. As Matco said, it HAS to fully retract or you lock pressure in the brakes.
I had installed a Matco parking brake in my lines as well, I worried that it might trap air or cause a problem. It did not.

 

[lewy]

Brakes

Wayne, did they say anything about pumping the fluid down from the reservoir to the bleeder valve,we have no restriction that way.

 

[yankee-flyer]

Latest from Matco

Matco says (below) that filling from the top doesn't lock the cylinder but doesn't do as good a job of bleeding. Van's says they've never seen this problem on the red airplane. As soon as I get more fluid I'll try disconnecting the cylinders from the pedal. I'm guessing there's enough variation in building the pedal assembly for some of us to have a problem. Now I'm wondering if that's the cause (dragging brakes on touchdown) of my excessive tire wear.

Wayne

From Matco:
If the shafts aren't fully extended, the bypass valve will be closed. You
will not be able to push fluid up from below. However, when the bypass is
closed, you can still push fluid from the top. (it operates like a check
valve) This feature is what allows the upstream cylinder to be able to
provide pressure to the brake regardless of the position of the downstream
cylinder (the pilot can provide higher pressure to the brake if needed than
the copilot is applying).

I guess I'm leaning toward the shafts of the cylinders not being fully
extended. That would be pretty odd for an RV since the pedal design is
pretty good. I'd be investigating that area. Start by disconnecting the
shafts of both cylinders on the left side system. Urge the shaft outward to
be sure they are extended (it would be nice to find the shafts 'pop' out
when the pin is removed from the mounting). Try bleeding from below. If
fluid now flows, you have found the culprit and need to determine the cause.

 

[DonFromTX]

And if it doesn't, then you have a bad master cylinder. Odd though that you would have bad ones on eiach side.
Regardless, if you cannot easily force fluid up from below, you have a problem that must be cured before calling the brakes airworthy. That problem would allow the heated expanded fluid to apply the brakes with no pressure on the foot pedal. Landing with locked brakes sure shortens your landing roll, but is rough on tires.