D

DanWright

Well Known Member
Just finished Sec 35 and am ready to fill and test the brakes. I read though the various posts on bleeding. They referred to top and bottom filling.
- What does that mean?
- Where are the instructions in the building manual?
 
Top Filling: put fluid in the reservoir and open the bleed valve at the brake to allow fluid to flow down through the system. Often drags air down into the tubing, requiring pumping of the brakes to get the air to rise out and/or extensive bleeding to get all the bubbles to pass through the system.

Bottom filling: use a pressure pump (often a pump type oil can with a tube attached will do, though I have access to a compressor driven filler which is very easy to use). The pump is attached to the bleed valve and brake fluid is pumped upward through the system. Done carefully, no air bubble are introduced. Insert a catch tube in the top of the reservoir to a catch can and when fluid starts overflowing from the reservoir into the catch can, switch to the other side and repeat the process. Then bleed a little fluid out one of the brakes to lower the reservoir level to allow for expansion and you are done.
 
Bottom filling

works best but quite a few of us have found that you simply can't force fluid up through the cylinders. The problem is that the last 1/16 inch of extending the piston opens a check valve and apparently a combination of tolerances can keep this from happening. If you can't force fluid up from the bottom, disconnect the cylinders at the bottom, manually pull them out as far as you can (probably not much more than 1/16"), wobble/shake them a bit, and the check valve should open.

Wayne 20241 143WM
 
The build instructions tell you to look into the brake manual for bleeding andthat's where the instructions are. Matco suggests two ways of bleeding. One is from the top with a vacuum pump and one is from the bottom with a pump thet injects the oil at the bleeding valve. I tried both techniques and found only the bottom pressure fill to work successfully.
See this post for full details. This post here shows why the vacuum technique did not work for me. In short, the bleeding valve allows air to get sucked in along with the oil, so the bubbles never stop. When you push the oil in from the bottom, the valves seep a bit but there's no air coming in.
I did not have a problem with the master cylinders. I did not have to remove the connection between brake pedals and the cylinders to do the bleeding like many others had.
 
If you have the described problem with the master cyls, you have a problem! What will happen later, is that you apply the brakes, fluid heats and expands, and the brakes will lock on if it cannot return. If you cannot force the fluid up to the reservoir, check your linkage, something is wrong.
 
DonFromTX said:
If you have the described problem with the master cyls, you have a problem! What will happen later, is that you apply the brakes, fluid heats and expands, and the brakes will lock on if it cannot return. If you cannot force the fluid up to the reservoir, check your linkage, something is wrong.
Click to expand...

What Don said is exactly right.
If you have a problem with the master cyls, that impacts proper bleeding of the brake system, that same issue will cause you operational problems as well (dragging brakes, etc,)
 
Ok, but

how do I fix that (and several other builders have the same problem, there's a thread somewhere from a year or so ago) without building another brake assembly-- and how would I know another one is right? When I described the problem to Van's support a year ago they said it would probably cause brake dragging but offered no suggestions. I don't think elongating the attachment holes would help, as the rod needs to be pulled down-- or will the hydraulic pressure take care of that?

With the cylinders attached to the pedals, even a standard aircraft one-man pressure tank filler can't force fluid through the cylinders from the bottom. You CAN bleed from the top, which is what I had to do the first time. At the annual last year (77 hours) MATCO advised disconnecting the piston and that worked. The travel after disconnecting them was almost too little to feel, but apparently enough to open the check valve.

I'm open to suggestions other than building a whole new brake assembly.

Wayne 120241/143WM
 
yankee-flyer said:
how do I fix that (and several other builders have the same problem, there's a thread somewhere from a year or so ago) without building another brake assembly-- and how would I know another one is right? When I described the problem to Van's support a year ago they said it would probably cause brake dragging but offered no suggestions. I don't think elongating the attachment holes would help, as the rod needs to be pulled down-- or will the hydraulic pressure take care of that?

With the cylinders attached to the pedals, even a standard aircraft one-man pressure tank filler can't force fluid through the cylinders from the bottom. You CAN bleed from the top, which is what I had to do the first time. At the annual last year (77 hours) MATCO advised disconnecting the piston and that worked. The travel after disconnecting them was almost too little to feel, but apparently enough to open the check valve.

I'm open to suggestions other than building a whole new brake assembly.

Wayne 120241/143WM
Click to expand...

Wayne, If you were able to bleed from the top, then it should also be possible to do it from the bottom. The issue can not cause a problem for one direction and not the other. My guess is that anyone that could not make one way work, but the other did, just didn't have the proper technique or tools (or both) for the way that didn't work.

If the system is bled but you are not sure if it is functioning properly, the best thing to do would be test it. Apply the brakes hard a few times. Then with the wheels off the ground, see if they rotate freely with pressure off the pedals. If they do, you are good to go.
 
JonJay said:
The simplest fix is to use one long bolt instead of the two separate short bolts. Run through both sides of the rudder bar tabs and rudder pedals in assembly with a long reamer or drill so they are then aligned and install the one long bolt. That fixed it for me.
Click to expand...

Jon,
This post is in the the RV-12 forum.
The RV-12 has a totally different rudder pedal/brake pedal design from the other RV's
 
It works!

Took a bit of doing but we found a liquid pump about half the size of an old style hand pump for balls. Filled the brakes easily, used about 40% of the jug I bought from Spruce.

No bubbles and brakes feel great. Thanks for the advice.
 
DanWright said:
Took a bit of doing but we found a liquid pump about half the size of an old style hand pump for balls. Filled the brakes easily, used about 40% of the jug I bought from Spruce.

No bubbles and brakes feel great. Thanks for the advice.
Click to expand...

I've been searching for a fill up from the bottom pump as you describe. Any more details???

Bob Bogash
N737G
 
NASA515 said:
I've been searching for a fill up from the bottom pump as you describe. Any more details???

Bob Bogash
N737G
Click to expand...

Auto Zone had the fluid pump we used http://www.autozone.com/autozone/images/part_type/0000001571.jpg Got some clear tubing from the hardware store that fits the brake fitting.

Details:
  1. Shortened the output line to a few inches.
  2. Inserted the conical adapter pointed end first into the output line. Secured it with a hose clamp
  3. Inserted (jammed) the clear tubing into the adapter
  4. Removed the inner piece of the brake bleeders and flushed them (found some small filings. Reinstalled them
  5. Filled one side until there was fluid in the reservoir
  6. Filled the other side and the reservoir.
If we messed up someone please speak up

The other key thing was a oil drip pan. http://www.autozone.com/autozone/ac...25yj?itemIdentifier=475418&_requestid=1546976 Sort of like a large cookie sheet. About $10. Keeps the mess to a minimum
 
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Has anyone tried a hand operated model aircraft fuel pump? I have one that is good for gas or alcohol...
 
Just last month ...

... I filled and bled the brakes on my bird. I used the " bottom up" method, which worked like a charm (except for the leaks at fittings I forgot to tighten fully!). On the recommendation of others here, I bought a cheapo garden sprayer from Lowe's for $8.98, and adapted the hose to a short length of Tygon that went on the caliper bleeder screw. I screwed a barb fitting into the fluid reservoir with a piece of 1/4" i.d. Tygon into a clear jug. I could sit on the floor next to the gear, open the hand valve on the pressure sprayer, and watch the fluid come out the reservoir (with or w/o bubbles) and drain into the jug. When there were no bubbles, I closed the bleed screw. Very effective and low cost method for brake bleeding.
 
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