Do some google search on how master works. Those inlet openings at bottom of reservoir are open in front of the piston when brake lever or pedal are free.

As soon a you hit the brakes, the piston travels past the opening and starts building hydraulic pressure.

If air is introduced to system, it is possible some will rise to master but down in caliper, there are areas that must be pushed or vacuumed out at the caliper bleeder. I use a small hand pump going slow and keeping vacuum low since the cup seal on the master will let air in from the other side if vacuumed too much.

I always one finger tighten caliper before coming off master lever or pedal if bleeding without vacuum..
I rarely do it that way except on new parts. Doing that way on old stuff puts the seals in areas that may have corrosion. When you do that, the master will fail in a short time. If you don't have a pump, only move lever a short distance like when you brake.

 

I have many years experience working with this type of disk brake system as an A&P...it is very common on small aircraft, in some cases right down to the components. I have never heard this method of bleeding brakes and it doesn't make sense that air would migrate up to the reservoir with the system closed in the power stroke. Pressure and vacuum bleeding from the caliper is the normal method however, it can be difficult to get a good seal on the stock bleed fitting when using a vacuum pump.

The method I have found the fastest and most reliable when I do a complete brake rebuild is to bleed the master first, connect the line and bleed it, connect the line and then bleed the caliper. Tapping on the caliper will help in moving trapped air. I have used this method with excellent results on airplanes, cars, and now my Harleys.

Also, DOT-5 can hold suspended micro air bubbles that will need to settle out before bleeding...had that happen once on my Dyna and it is a PITA

 

Something missing is what is happening. We池e all taking guesses, but we don稚 know the phenomena you池e experiencing. So a number of us are undoubtedly going down a wrong track.

so how about telling us, with details, what is going on.

 

Only time ports are open is when brake is off. You misread what i said or it's too rambling to understand. You have not been following along. That thing about some air coming back was above my post, it was brought out. I said some may come out but was not an acceptable way since air would stay in areas in system. And you are correct, some air does come around threads in bleeder. However it works. You just got to make sure you bleed enough since you will always see air in bleeder jar.

I use vacuum on old stuff to change out fluid. New stuff, exactly like you say.

 

Yes the reservoir is blocked off while the master is in the power stroke, this has nothing to do with why I suggested to leave it in the power stroke overnight.
If you have stubborn difficult to get air stuck somewhere, then it will be easier to force it out with pressure vs vacuum. And if you leave it under constant pressure, it will usually work itself along the system to one of the highest points instead of remaining in it's stubborn hiding place. If it works itself to the caliper a standard bleed in the morning will release the culprit air. If it works to the master, as you release the master the bubbles will travel up into the reservoir. If neither of those happened, it is likely stuck at a mid point that is slightly higher, such as a pressure switch.

This method is not often required if you bench bleed the replacement parts and bled the system as you reassembled it, then did a final bleed. But sometimes it is, or sometimes you rushed and didn't bench bleed at all or thoroughly enough. I have never had this method not correct a stubborn air trapped issue. It has worked on every motorcycle, car, truck, airplane or helicopter I have ever worked on. And most of the aviation systems are far more likely to trap air somewhere than any motorcycle system could dream of.

 

Make sure you wheel is properly aligned and that your pads aren't worn at an angle.

 

When you open a bottle of side it foams up a bit, and you can watch the bubbles of carbon dioxide forming and rising up and out of the soda.

Why?

Because the carbon dioxide is suspended in the soda under pressure, and released when the pressure drops.

Same with air bubbles in brake fluid. Leave the system under pressure and much/most/all the air can be suspended into the brake fluid, just like carbon dioxide is suspended in bottled soda or beer. Now when you bleed the system the suspended air comes out.

Uniform pressure won稚 move an air bubble in any direction along a hydraulic line. It痴 directional flow and gravity that will move it along or up.

 

Hey guys , thanks for all the great answers , it痴 been fixed with a new master cylinder !!!! Me so happy 😀

 

Thanks for updating us on the proper repair !

 

Wow ...5 pages and 47 replies to figure out it was the master cylinder...which was diagnosed in the first reply to the thread...

If you ever suspect the MC...crack the bleeder open...not rocket surgery...