A dial indicator is the most precise way,but if one is not available,roll the wheel slowly while looking where the rotor is in the caliper,it is a good chance when the bent portion of the rotor reaches the pads the wheel will get harder to rotate,and you will see the rotor push the pistons back in their bore,that is where you will lose pressure at the lever.

 

If you have a warped rotor your you won't lose pressure and get a soft lever, you will feel the lever walk in and out as the wheel rolls and if it/ they were warped you would have knower it before the rebuild.
You either have air still in the system or the fluid is bypassing at something.

 

When you lose the "pressure" can you pump it up and then the pressure is maintained without moving the bike? Let is sit in the same spot and the pressure is still there when you reapply the brakes? If this is correct and you've checked the rotor/s for runout I would consider checking the front wheel bearings. If the bearings are bad/loose enough to let the wheel "****" this would/could cause the piston/s to be pushed back into the caliper bore ... Just a thought.

 

I ordered some new rotors. I'll keep everyone posted

 

Did you verify that they were warped?

 

What model? If you have a model with ABS, you'll need a dealer to bleed them - gotta have Tech II to activate ABC properly for good bleeding.

 

Curious about this also. I think youre wasting money on something that doesnt need replacing. What year is the bike? Did you change the fluid to another type?

 

Here is an example of why you will lose lever pressure with a bent rotor.Have you ever changed your own brake pads or put a wheel back under your bike after a tire repair?most times you will spread the pads (pushing the pistons back into the caliper bores)To ease the installation of the wheel or caliper.After this is done the brake lever or pedal must be pumped several times to regain brake pressure before riding the bike.In essence this is what a bent rotor will do every time the bent portion passes through the caliper.I have witnessed this many times.

 

I think geezer glide has it right.
I've experienced this when changing pads on cars.
Sad to say I've never kept a bike long enough to need new pads.

But what he is saying will absolutely cause the issue.

 

Youre not building pressure when changing pads, just moving pistons bA k up against the rotors. If the rotors bent the pistons will move in an out or pulsate when applying brakes. Its not a rotor he has an internal hydraulic issue going on.
Whyy did you go into the brakes in the first place? If the problem arose after you worked on it, there in lies your problem, not the rotors for sure.
You still have not stated what year the bike is and if its ABS or not, nor if you switched to a different fluid. If you are seeking help here, you need to state everything you have done and for what reason, in order to get an accurrate feedback on where you need to look at.