48Big is correct. This almost *has* to be master-related, if you are not getting pressure or fluid to the inlet of the proportioning valve.
Do you get fluid flow out of the line from the master if you disconnect or loosen it at the prop valve? If you do, do you get fluid flow at the outlet side of the prop valve if you loosen that line? You might go to the FLAPS and get a couple of inverted flare plugs and try isolating things. If you plug the outlet of the master cylinder, you should immediately get a high, firm pedal. If you do, the trouble is further down the line. If you don't, the trouble is in the master. Same thing applies if you plug the outlet of the prop valve.
Keep in mind that the only place in the system where fluid can move without leaking is in the master cylinder. If fluid moves anywhere else in the system without doing work, it has to leak out. (Well...you can have a case where the master and the prop valve fail in just the right ways at the same time. If this happens fluid will pump from one compartment of the reservoir to the other.)
Hmmm....you say, "...all this happens after it has been parked all winter..." Is this a repeating problem, year after year, and if so what did you do to fix it in the past? If you were able to correct it by bleeding the rear brakes, then it may be the proportioning valve. One of the functions in a drum/disc combination valve is the residual valve for the rear wheel cylinders. The function of the residual valve is to keep air from pumping past the seals into the wheel cylinders, so if the valve is not working you can get air into the cylinders. In that case it would be time to replace the combo valve.
The Truck Shop in Orange, CA (800-243-8947)has them. $50 for a GM replacement, $80 for one that looks like it might be an OE style.
Joe
