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Brake Reservoir Fill Quantity

Karetaker

Well Known Member
Howdy everyone. I plan to add/bleed brake fluid soon to my RV 7. (pressure fed from bleed screw at caliper) It is not clear to me how full the reservoir should be when finished. Full, not quite full, half full...?

Much obliged.
 
In my experience, they lose little fluid over time, but others have different experience. I only fill to 1/2 to avoid spillage. I tend to look at the clear tubes in the cockpit before each flight to identify leaks and this would also tell you if the reservoir has become empty.

Larry
 
Howdy everyone. I plan to add/bleed brake fluid soon to my RV 7. (pressure fed from bleed screw at caliper) It is not clear to me how full the reservoir should be when finished. Full, not quite full, half full...?
Much obliged.

I usually leave about 1/2-5/8" at the top. Main thing to remember is when changing brake pads, you will push more fluid back up into the reservoir. So you may need to remove some fluid at that point.
 
I'm a 1/2" from the top kinda guy also. A little air space never hurt anyone and if you get a leak that extra fluid is going out the hole anyway.
Fixit
 
I completely fill it, then dip a zip tie to displace just a little bit. It seems that the level ALWAYS goes down, even with no leaks. Don’t know where it goes, but every plane I’ve serviced, this is true. Then whenever the cowling is off, I dip the zip tie, if nothing approaches the top, I refill and displace the excess.
 
It seems that the level ALWAYS goes down, even with no leaks. Don’t know where it goes,

As the pads wear, the piston has to move farther out to make up for the wear.

The piston has no return spring, so it just stays a bit farther out and does not return to fully seated in the caliper.

This motion uses up a bit of volume that the fluid makes up.
 
As the pads wear, the piston has to move farther out to make up for the wear.

The piston has no return spring, so it just stays a bit farther out and does not return to fully seated in the caliper.

This motion uses up a bit of volume that the fluid makes up.

With cars, I’m familiar with this. But I’ve noticed even if a plane sits for two years, no brake wear, yet the fluid seems lower. Evaporation maybe? Idk.. I like to fill to the top. Running out of fluid when you need brakes can be bad.. I don’t see any benefit to having anything other than a full reservoir.
 
This is why I'm so glad I bought one of the clear acrylic (or whatever) reservoirs...easy to see on pre-flight that I have plenty of brake fluid.

About 3/4 full has always worked fine.
 
Brake fluid Reservoir solid cap top

As I have an inverted system on the RV6 and use it, I have a solid cap to prevent leaks I keep it 1/2 full to give it some air to expand and contract , I check it more often with a white tie wrap
Peter
 
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