First time joining a forum so I hope I am posting this correctly. I have always searched and found good info using these things but have finally ran into an issue I cant seem to find anything about. I have a 1986 F350 6.9 IDI SRW LWB. I have had this truck for a little over a year. Since I bought it It has had braking issues. Seems like the front disc brakes are not engaging properly. Early on it was intermittent and for the first few stops they worked just fine. after stopping 3 or 4 times they would stop working and I am stuck with just the rear drums. After a few months they stopped engaging all together and only rear brakes seem to be working. To date I have changed in order vacuum pump, vacuum line, brake booster/master cylinder, brake hoses. The weird thing is every time I changed out a part it would work again briefly. I thought maybe there is a piece of trash floating around in the proportional valve? So I pulled that off and apart and cleaned it. Very hard to get to by the way unless you take the time to pull the fender well, which I didnt. I didnt see anything abnormal except for maybe a worn out O-ring. With out a new one to compare it to I put it back together with the assumption it was ok. After bleeding all the brakes again it worked! or so I thought. they worked for about 10 times then quite again same as always. I have a new proportional valve sitting on my work bench to be installed next chance I get. Is there anyone out there that has experienced a similar issue, or has some input and would be willing to share some words of wisdom? I have been chasing this gremlin for a while and am ready to have it resolved.