[Vwdiesel] Air suckers?

[Sandy Cameron]

I have had a strange phenomenon with the brakes on the A2.

It's been a years or so since I did them all over, incl drums, rotors, shoes
and pads.
Blew out all the old fluid by blowing in new stuff. 

They worked great all year, then started to get low. I thought it might be a
stuck adjuster and pulled both drums and checked. Linings were good, and
adjusters seemed to work ok, and when I put it all back together, they were
still low.

So I decided to bleed the left rear, and sure enough, got air out of it.
Tried the brakes, just like new. Drove for a month, they went soft again,
bled both rear wheels tonight and got a little air from both. Brakes are
fine now.

Where is it coming from??

The reservoir is full, has never lost any fluid, no puddles on the ground,
pedal is firm and does not "creep".

Yet it's as if air got in to the system somehow, and only to the back brakes.

Is it possible air could work back past the rubber cups in the wheel cylinders. 

They are pretty old. perhaps there are pits that hold air when relaxed, and
with application, the cups move out past them and air escapes into the cyl.

I doubt this, as the pressure of application is about 2500 LB sq in.

If it happens again, I will replace both cylinders.

Is it possible the "equalizer" valve might let air in under certain
conditions? This one has been seized for years.

Sandy