2004 Ford Van INTERMITTENT Brake Problem

On 3 occasions over the past 3 months the brake pedal on my van has gone right to the floor. The first time I took it to a local shop to be checked out, they found nothing wrong, no leaks, they bled the breaks, etc. The second and third time this happenned, last week, I took my van to a local Ford Dealer. They also checked it over and found nothing wrong. The dealership is recommending either replacement of the MASTER CYLINER at $400, or replacement of the HYDRAULIC CONTROL UNIT at $850; they have made it clear that there is no way to determine which is the problem. I basically have to GUESS which one is the problem, and if I guess wrong I have to have the other item installed. Is there anyone that can give me some solid advice regarding which item is the most likely problem.

Thanks ever so much for your assistance.

Dennis from MN…

(this is a follow-up from an earlier question, thanks to all who responded earlier)

If it were me I would disconnect the lines at the master cylinder, install plugs in the master cylinder where the lines were and press the brake pedal down (not hard but with steady pressure. If the brake pedal falls while you are holding pressure on it the m/cylinder is leaking past the seals internally. Cannot tell you much about the control unit. A thinking tech should have thought of this, after all, I have not swung a wrench professionally for 21 years but I remembered. The dealer will charge to do this since it is there job to gig you.
Any dealer techs reading, save it! I know what I am talking about since I am working at a dealership now.

A dealer wants your money bad. Try some other places and let them change the master cylinder if you can get it done for about $150 less. Yes, you do have to guess because most mechanics try to make you do that. I couldn’t get a diagnosisfrom a mechanic, so I had to find the problem myself a few times. I wemt to more experienced people and had to become more experienced. I have had a good record. Now I have to step up another level. Again! It costs more every time.