Assuming no leaks and all new rubber in brake system, pedal should be rock hard w/o servo. With servo there will be a slight movement and pulse felt on pedal when initially depressed with engine idling.
For occasional driving, I highly recommend silicone. All brake fluids present problems in our cars. There is no perfect fluid. Silicone, in my opinion is the lesser of all evils. I do not want to discuss fluids here.
The issue is spongy pedal. Silicone fluid requires special care when installing and bleeding. Done properly, you will have no problems for a long, long time.
The first thing NOT TO DO. is "bleed the brakes vigorously". It’s just the opposite. When bleed vigorously, the fluid, any fluid, will tend to cavitate when going through orifices or internal restrictions. This creates a momentary negative pressure on the fluid.
With silicone, the problem is more acute in that the fluid vaporizes at a lower vapor pressure. That is, it turns into a gas, as in air bubbles in the line. The fact is there is residual gas (air) in fluids. That is why they tell you to keep the containers closed at all times. Anyway, If you still have spongy brakes. I’d begin by bleeding our all the fluid. Save it if still clean. It is possible to remove the gas from silicone or any fluid by placing it in a vacuum chamber and pulling the air out. Most owners don’t have access to this. So new fluid is the easy way to go. A pint to pint and half should be sufficient.
So how to bleed. The simplest way if you have the time is to allow gravity do it for you.
1. Close all bleeder screws except one, any one. This screw should be open about 1/4 turn with hose connected into a container.
2. SLOWLY, pour fluid in reservoir. Do not allow bubbles to form.
3. Watch fluid, if it does not go down, prime system by slowly pushing master cylinder (brake pedal) in all the way. VERY IMPORTANT, SLOWLY, VERY SLOWLY, allow pedal to return. About 5 seconds. Remember, you do not want to aggravate the fluid at this time. It may be necessary to do this a couple of cycles. If impatient, you can continue this process til fluid goes out the bleeder.
4. Allow about one reservoir volume to gravity feed through bleeder screw. Assuming fluid is clean, recycle in to reservoir.
5. Close first bleeder screw, repeat gravity feed for remaining three screws.
Obviously the reservoir must be kept full all the time or you start the process all over again.
I have found that I have less problems by being patient and using the gravity approach. There is a tendency when pumping the master cyl to go too fast.
This entire process takes about an hour.
Having a second operator open/close the bleeder screw while the master cyl is being pumped works against you with silicone, any fluid for that matter. Its just that Silicone is more vulnerable. With the screw closed, a vacuum is created in the brake line. The vacuum along with resident air is mixed with the fluid in the line because of turbulence while moving as the master cylinder piston is being returned to its parked position. Now you know why silicone can be spongy.
Silicone does cause rubber bladders in the brake light switches to fail . Count on it. Adapt a mechanical switch to the brake pedal arm just like all new cars do.