How many miles have you run this system so far? I'll agree with Troy's thinking that you may still have some air that needs to purge out of the PS system yet, though this would not be the case if you have more than 500 miles of road use already accumulated (the air should be long gone by this time unless you have a very poor PS reservoir design). Based upon your description of what happened, I'm theorizing that you may have been part of the way into an unexplained engine stall condition = pressure drop from the PS pump due to being just about stalled out / possible PS pump hickup (pressure control valve in the pump may have stuck in its bore momentarily). Your comment about the pedal going to the floor has me thinking that you may also possibly have a different problem somewhere downstream of the brake assist unit, as these assist units will always provide a true manual brake connection between the brake pedal and the master cylinder = even under a worst case scenario, you would have still had the manual brake backup function (impossible not to have the manual brake function not available at any point). Note that we design our systems to connect to the manual brake point on the brake pedal in the 1st gen GM F body systems, which offers an added measure of leverages mechanically available for use in the "worst case scenarios". When this condition happened, you went into a state of pure mechanical backup in the assist unit: input piston strikes the output piston which provides a true "manual brake push through compliance" + also using the higher brake pedal leverages available as connected to the brake pedal = under no circumstance could you be any worse off than a true manual brake scenario (atleast as far as the brake assist unit is concerned). When the PS pump stops producing pressures (like when an engine is about to stall or substantial PS pump cavitation has occurred), the brake pedal height may drop about 1-2" lower than usual in some instances (until the accumulator / reserve power function kicks in), though by no means would allow the brake pedal to go to the floor unless there was a pressure loss somewhere else in the braking system. Upon restarting the engine, everything went back to normal, so time will tell if you only had a temporary hickup of sorts or whether there may be a further issue. Click here for good reading regarding a mysterious loss of brake pedal on a 1st gen Camaro, as this may also apply to your case (or not):
http://www.hydratechbraking.com/foru...opic.php?t=135
Let me know what further details may be possibly involved, as it seems that there is always more to the picture than what is absorbed at first glance.
What type of PS pump / reservoir are you running?
What type of PS fluid?
Have you reblead the brakes since the initial build yet?
What steering gear are you running?
Is the engine carb'ed or EFI?
Any recurrence of this scenario again at any point?
The more info you can provide, the better we can all do to help nail down what the issue may be...