Dodge Repair: 03 Minivan: intermittent spongy brake pedal, wheel brakes, dodge caravan

Question
I have a 2003 Dodge Caravan.I am experiencing "inconsistent" brake pedal action.It seems fine at times with good pressure and than seems to have a soft lower pedal.No leaks.No loss of fluid.I have heard there were(are) some problems with the master cylinder on these vans.Can you give me some suggestions for diagnosing exactly where the problem might be.Low mileage 145,000 km.No anti-lock as far as I know.Any suggestions on how to replace  the master cylinder if you think that might be the problem? Thanks!

Answer
Hi Mark,
The replacement of the master cylinder is a lengthy process which would be best shared by copying it from a CD and attaching that to an e-mail I would send to you directly. But there are less difficult and expensive things to consider first:
At any time in the past was the fluid in the master cylinder allowed to run low? If so, then bleeding the master cylinder for trapped air would be a good place to start. Otherwise I believe that you would do well to bleed the brake lines in the following order to remove trapped air if there is some in the system: left rear/right front/right rear/left front. Be sure to open bleeder valve a full turn and press the pedal down 4 times (closing valve before releasing pedal slowly) and collect the fluid which can be reused if clean and not too old. Have enough fluid to not allow the level in the master cylinder to run low.
May be even before that you would do well to check the vacuum hose between the power brake canister and the intake manifold as if there were a leak in that line or its connections at one end or the other the sort of thing you describe would be experienced due to power brake function being impaired (term is called 'running out'#.  
And if you have drum type rear wheel brakes it would be good to adjust those shoes.
I suspect that one of those three possibilities #trapped air/impaired vacuum/rear shoe adjust# is going on in you vehicle.
Please 'rate' my answer #see below#.
Thanks,
Roland