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on my 68 fargo I have installed newer 1990 one ton dana 60 diffs front and rear. the brakes ran fine for a couple years and last summer I was having issues with the front disc brake pads falling out on passenger and driver side. the brakes would be firm and feel great about half the time and some times it would go to the floor. doesn't matter if you pump it up or not. rotors and pads are in spec. at that time I had a factory 68 brake booster installed with the factory proportioning valve removed. so I started by replacing both front calipers with new units and replaced the master cylinder 2 times with a remaned unit and once with a brand new unit for the 1990 year. that didn't fix it so I figured maybe its a bad booster so I took that out and its now manual brakes that didn't help. so I figured maybe its the back brakes so I upgraded the rear brakes to disc with a weld on kit from cut throat 4x4. they work great but the pedal is still only firm half the time. so next on the front brakes I upgraded to a new stainless steel flex line and I replaced the steel line from the master to the flex line and I added a 2 pound pressure valve hoping it would keep pressure on the pads and I also replaced one steel line on the axle. still have the same issue. only thing I have not replaced on the entire brake system is one steel line on the axle and it does not have any visible leaks at all. the entire system does not have any visible leaks and I am not loosing fluid in the master. when I bleed the brakes all the big bubbles come out but I will say there are some very small bubbles left in but I don't think it would cause the caliper to open up about an entire inch to allow the pad to kick out. its the outer pad falling out and it has huge mounting tabs that wrap around the caliper.
just wondering if anyone has any other ideas or if there is something im missing because im out of ideas. It depends sometimes I can drive for ten minutes with perfect brakes then all of a sudden it goes to the floor and only the back brakes work.
thanks.

Excessive wheel bearing play?

wheel bearings are in spec. im almost wondering if its a bad master again. I don't know what the odds are of having that many bad masters. I bench bleed them and they bleed very good. the one master I did put in had a bad rear seal from new. maybe I should try a different brand altogether. it just blows my mind how sometimes the brakes are 100 percent good then sometimes it goes to the floor and only the rears work.

Yeah that is really strange. The fact that it is intermittent almost sounds like it is related to something moving while you are in motion (like a flex hose kinking), but that should not allow the pads to fall out (especially on both sides). And the fact that you added a 2lb residual valve pretty means that the caliper pistons shouldn't be pushing back that far anyway. I'd say the calipers and discs might be mismatched but you mention they worked OK for a few years. Is it all Mopar parts and the MC is plumbed normally (front reservoir to back brakes, rear reservoir to the font)?

Just a thought I have... Back a few years ago we were doing a Toyota truck caliper conversion on my son's Datsun 240z and I put the calipers on the wrong side. The bleeder port was on the bottom and should be at the top of the caliper. That caused him similar issues until I put them on correctly...Just a thought without pictures of the install that's all I got.

yup it is really strange, I have many mopars and have never been stumped like this. yup the parts are all factory mopar style and resemble the 4 other dana 60 fronts I have. front reservoir goes to the rear brakes and rear reservoir to the front. I have the residual pressure valve mounted a few inches downstream of the rear reservoir. and calipers are on their respective sides. there is no proportioning valve. the rubber hoses coming off the calipers do rub on the shocks a bit but they are newish hoses and don't seem to be kinked and no cracks. I put the residual pressure valve in about a week ago and I haven't had a pad fall out yet per say in the hour or so that I drove it. but the pedal is still doing the to the floor thing off and on.
its kinda getting enoying trouble shooting this lol because its kinda hard to drive around a huge mud truck on super icy roads with potentially no brakes haha.
and thanks for the replies.

well I talked to a few guys at work yesterday and they were quizzing me and I think I know what it might be now. when I changed it over to manual brakes my brake pedal push rod didn't line up perfectly with the piston on the back of the master. sometimes its binds up just a hair and puts about a quarter inch of stroke on the master. personally I didn't think much of it and thought if anything it might help the situation by keeping some preload on the brake system. but they were saying nope it doesn't allow the proper pressures inside the master. so when I get home in a couple days I will see if I can line things up or clearance some things or maybe a return spring.
I hope that's the fix well anyways have a good day!