Definitely backwards.

 

But guys.... Right after I posted that pic I realized I'd set up the photo wrong and edited the text to describe the situation. The cup wouldn't even sit against anything if installed as shown in my photo. As noted in my edit text, the brass ring embedded in the cup goes up against the end of the piston with the tiny holes around it's perimeter. So, anything else it could be? It was definately installed the other way around. I know this stuff is very confusing when someone posts a photo and they didn't even get the photo correct. Sorry. Whatever is wrong, it makes a nice parking brake the way it is...... Here's how the piston meets the cup.

 

Been a while since I worked on one of those.......but I seem to recall the filler cap has a tiny vent hole in it? Could be plugged?

Just guessing here, as I'm not totally awake yet. (don't ask)

 

Already ran a wire through the two holes in the square nut on the filler cap. No resistance, but still I shot 100 lbs of air through them..... I'm narrowing it down to the check valve. I understand it's necessary for a vehicle with the MC mounted below the height of the wheel cylinders so they don't drain back into and through the MC. Remember the first component placed in the bore. That 3/16" thick rubber seal/grommet piece. It makes a very effective seal. I don't see the part in either of the two schematics presented here. Could be that it was originally bonded to the face of the check valve as one part. Could be it shouldn't be in there, and is both increasing spring pressure as well as too positive of a seal. I have been saying the brake system acts as though there is a check valve in it that holds all the pressure that's been applied. How can a check valve that has a flap inside ever release the pressure? I can blow through it from the spring side, but not at all from the back side. It seems that it is just plain a check valve. Should I try it without the rubber grommet?

 

I might be confused(likely) but the above pics are for 48-52 trucks and yours is a '47. Might be some differences......
Looking at google images, some show the rubber washer, some don't. The truck kits seem to include it.

Ford Master Cylinder Rebuild Kit - 91T-2004 - C&G Ford Parts

Maybe try it without? Ain't old trucks fun?????

 

Thanks for the link! Yes, I see it's in there. Hmmmmm Fun??? It's about 95% fun and 5% pure frustration. Most of the guys that have worked for me over the years have been avid sports fans. They can't understand why I spend my free time all greasy, under a truck. From what I see, they are frustrated about 50% of the time, or whenever their special team is losing. I'll take my odds any day! Since the disc check valve seems to offer complete resistance to the back flow of brake fluid, I'm thinking the long spring allows the valve to lift off the rubber seat, thus eliminating the excess residual pressure on the brakes. So the long spring is too strong????????????

 

There should be nothing behind the valve. The rebuild kit from C&G supplies 2188, a new stop washer. No breakdown for the 47 or newer 48-52 shows anything behind the valve.

I would wager that if you remove that gaskety liike thing from behind the valve, your MC will be fine. If you still have brake issues, I would then begin to suspect the hoses or lines.

 

The frustrations you're having is the reason I quit rebuilding brake components and now buy new or rebuilt.

 

Couldn't agree more, Ray! I rebuilt the brake parts on my first two trucks when I was in my early 20's (40 yrs ago). Then I quit in frustration...... On this truck I put myself on a tight budget because of the recessionary times in the building sector. Things are steadily improving and next time it's new stuff! BUT, here's how it went at lunchtime today. I talked to my favorite parts guy, Bob Selzam and he said the piston is the right length. However, he said there are two holes from the res down to the piston. One large one about 1/8" in between the two cups and another just after the primary cup which is much smaller. I looked down the fill cap and saw this in plain view. It took a search on google images to see close up. From inside the master it's about 1/16th dia, but tapers down to very tiny. If this is clogged, fluid can't return to the MC. I couldn't get a piece of .30 welding wire through it, and finally settled on a sewing needle in a pair of vice grips. Success! Then I chased it with a couple of torch cleaner serated wires. Re-assembled and drove with lots of braking and it's perfect. Still gonna buy new or rebuilt next time. With the amount of shop hours I have missed out on, I could have bought enough new brake parts for two or three trucks. Just couldn't let this thing beat me.... Have a great July 4th weekend. GB

 

Excellent news! Glad you found it. Have a great 4th.

 

With all the past over the past year or two about these brake boosters, and the trouble I had, I wondered why my booster looked so different from some of the ones posted. When this thread started I started doing more research to find out which booster I actually had on Edith.
Well, it turns out that I don't have a Midland booster at all, but instead a Bendix piston type booster. The book calls for the 8T-2005, but what I have is the B7QH-2005A. This I'm sure was changed sometime during the truck's life as this booster was first used in 1953 on the C 800, F900 and the T700 tandem.

 

These trucks have had about 60 years to be cobbled up, repaired and re-invented. Seen a lot of swapped out parts and done quite a few myself. Someday someone will wonder why it sounds like my driveshafts have a bunch of BBs in them. As for my brakes, now I'll need a new subject to ponder when I wake up at 3 am to pee, and can't get back to sleep.......