WRT a shower base, I am not familiar with the
term "receptor", or how it could be lifted up.
But, he base is made of acrylic and is very rigid.
If you have seen a regular fiberglass
shower bases at say your friendly neighborhood
home improvment store, then this base would be
similar. Unfortunately, the mortar has hardened
and the base has enough contact to be firmly
secured. However, there are these air pockets
that are between the mortar and the base in some
places, so when you step on them you get
a sinking feeling. The base manufacturer
states that such a condition
can lead to cracking or other problems after
prolonged use. I wanted to fix it now while
the bathroom was still unfinished.

The area near the drain is well attached, so I
cannot inject any substance there. However, I
do have access to the underside of the flooring
and could drill until I encountered an air pocket.
For each pocket an inlet and outlet hole would be
required to replace the air with something.
This is very precarious, since if I drill too far
then I will drill through the base itself, I have
a whole new set of problems.

One of my questions, is does any one have a
better solution than drilling or a different
technique on drilling?

The other is what substances would be good to
use as a filler? Silicone caulk? More mortar
thinned using water and injected?

Jim