Ur-q brake questions

[thompers@together.net (Thompson Smith)]

Greetings,

Here's a good one for you Ur-q owners. Please help me figure this one out.

My 1983 Ur-q has been upgraded with front Girling 60 twin pistons,
Metal Master pads, Brembo drilled rotors, new master cylinder (2yrs ago),
ATE Super blue fluid, and stock rear brakes.

I drive it everyday, commuting around 40 miles. This last summer I
recently participated in the QCUSA Lime Rock track event, and a local
hillclimb here in VT. Just before this I installed all of the above.
(I do have a very small leak in the power steering rack. This only needs
topping off every 2000 miles or so.)

Here's the problem-

When I let it sit for any period of time longer than say 6 days, and
then go drive it, the pedal slowly goes all the way to the floor when
first trying to brake... but, if you shut off the engine and pump the pedal
(to relieve the bomb pressure & check PS fluid level) 20 or so times,
the pedal becomes rock solid. This would tell me that the master cylinder
is still fine.  However, when the engine is running and PS pressure is
working, the valve under the bomb must be letting something by?  BTW,
it has worked flawlessly on the track and everyday. This only happens
when it sits for a few days.  (I know, I know, drive it, don't let it sit!)

Wondering mind.
Regards,
Thompson Smith