I am having a problem with air getting into my IV-P brake system.

There is and never has been any fluid leak.

After I bleed the brakes, the pedals are not spongy.  But after just a few
flights they do get spongy and continue to get worse.

When I bleed them I'm using a pressure pot attached at the caliper. And I
have a hose fitting in the top of the reservoir with a clear hose running
into a can.

After bleeding, I pump the brakes three or four times and I start to see
very large air bubbles appearing in the clear hose coming out of the
reservoir.  I have re-bled and re-pumped up to 12 times and the same thing
happens.  Because the air appears so quickly (i.e. with three or four pumps)
at the reservoir I believe that air is getting into the system from the
master cylinders.

I have overhauled the pilot master cylinders.  If anything, the problem
seems worse.

I do not use the copilot master cylinders.

I have overhauled the calipers.

The problem occurs on both the left and right brakes.

The sponginess in the brakes seems to occur more quickly at higher altitudes
under pressurization.

The calipers are not rubbing on anything when the gear is retracted and let
down.

The vent on the brake reservoir is clear and open.

Technical Support at Cleveland says the problem could be that the rod length
on the master cylinders is not adjusted properly or that the master cylinder
may be defective.

I'm not sure what to try next.  I'm thinking of eliminating the copilot
master cylinders temporarily and putting those cylinders in place of the
pilot master cylinders just to see what happens.

Has anyone else had this problem?

Any input is appreciated.

Bob Comyn
N430BC - 165 hrs