Firstly you need to confirm that is the problem. if you are running rubber brake hoses (I think you dont from memory) you would clamp the hose using brake hose clamp and then check your pedal feel, and to further confirm and
if you run braided you will have to physically have a look if pistons are moving an excessive amount(remember it doesnt take a lot of piston movement as there is 8 of them just in your front brakes. you may need another person to assist(someone pushes while you have a look).
If this is in fact excessive piston pull back, just so you or anyone else reading fully understand how it works, why it is happening and how to fix it here is a quick explanation.
Piston pull back or Retraction works like this - the caliper piston is sealed by a seal with a rectangular cross section. When the piston in the caliper is applied this seal tends to "stick" to the piston and it deforms sideways as the piston moves. When we let off the pressure in the brake, the seal wants to regain its old shape and it retracts backwards and drags the piston with it.
This is a normal operation, however when the seals are new(freshly rebuilt) or possibly not lubricated enough this action is so excessive to a point of almost no pedal which responds to couple of quick pumps resulting in full solid pedal that doesn't creep down(if it creeps down it may be another issue, master cyl etc.). The problem is much worse with multi piston calipers as more fluid is displaced making the pedal lower/worse.
Now that you understand how and why here is how to fix it. If you had some pull back but reasonable pedal this problem would normally go away with a bit of use, few thermal cycles etc and pedal would become normal. Since it is very bad this is what you have to do.
The idea is to make the pistons come out further than they need to be, then push pistons back just enough so you can slide the pad in with some friction/resistance. An ideal tool is partialy worn pad, slide you worn pad in and pump the pedal till pistons contact the pad, now remove worn pad and replace with new pad making sure there is friction as you slide it into caliper! you may need to repeat the procedure till you get it right. once happy move to the other side of that caliper and then do the same with the other caliper. After a bit of use the seals relax enough so that this no longer happens, although I did have a bad one in the past where I needed to re-do above procedure after the bed in period.(was a multi piston/small master combo)
The reason for using an old pad is so that you dont accidentaly 'pop' the piston.
Before you go ahead and do this, make sure that this is the problem by confirming first.
Cheers