Anyway, here's my guess.

So balancing the forces you get (Internal - Ambient)*Area = M*g and ambient is considered constant. To increase M means you must increase P.

According to PV=RT, this is done either through decreasing volume or increasing T. To decide which it is, we must consult some laws.

The 1st law involves enthalpy which involves temperature. I don't know if I remember it exactly, but here goes.

I believe its Win - Qout = h2 - h1 = something.

The Qout is zero because it is an insulated piston. However, there is boundary pressure work being done. Work is a N*m, pressure being a N/m^2 says I probably need to multiply (Internal - Ambient)*Area*Distance Compressed. Which indicates a volume change will probably occur. Also, the work into the system will be positive so the volume will decrease. I guess Area*dheight is a dVolume? So the boundary work is PdV.

So PdV = h2 - h1 = cp(T2 - T1)? So for a dV to occur, a temperature change must occur. The left hand side is positive for this equation so T2-T1 > 0, suggesting temperature increases.

So as you add mass, the volume decreases, pressure increases and temperature increases.

EDIT maybe I should check the 2nd law to see if one of those remains constant. It's an isentropic process so ds = 0. Lemme think.

EDIT EDIT and where is this temperature increase coming from? Energy of the system remains constant. Well, I guess the energy of the system will change. The piston has some potential energy which is altered for a kinetic energy which will be passed into the fluid before equilibrium is reached again.

I'm gonna check out my book to remind me the 2nd law.