Help! Power drum brakes!

[jescates]

So after sitting 5 years, decided to get my 70 road worthy! So I changed out the drum shoes, wheel cylinders and rubber lines. I bled and adjusted everything! Brakes are still a little mushy, but I can live with that! The problem is, today I drove it for about 30 minutes, when I went to enter the driveway, I hit the brakes and it slowed, but when I hit the brakes the second time, the brakes faded to nothing! Had to use the emergency brakes to stop! So, any ideas what to change or adjust? I can pump the brakes a few times and the brakes work, but then it fades to nothing! With the engine running, I pulled the valve off the brake booster and air rushes out, so I think the booster is working. Can a master cylinder be the culprit? I checked the fluid and it's good, it's not leaking or anything! Getting a little frustrated right now! Thanks for any insight!

[Rannoch]

No fluid leaks? Master cylinder is good?
You probably still have air in the lines.

[jescates]

Bled them twice, nothing but fluid coming out. No leaks so far and the master has the same amount of fluid. The brakes will pump up, but if I hold down the brake in gear, it slowly fades to nothing and starts rolling!

[hamjet]

I think the first thing I'd do if it were mine is pull the wheels and drums off and make sure the new wheel cylinders aren't leaking. It could be the MS, did you have the same problem before you put new parts in?

[jescates]

The brakes were working somewhat. I came to a hard stop and had to lock them up. After that, the brakes were mushy. So I checked the reservoir and it was almost empty. That led me to change the breaks and of course I found a leaking wheel cylinder. So I changed all 4 wheel cylinders and the front rubber lines. When changing the rubber lines, I did drain the master cylinder! So maybe it needs bleeding or replacing! Everything is new except the booster and MC. So if I have to pull the MC, I might as well change it out!

[firedemon]

mine did the same thing I replaced the master and it works great now

[70STOVEBOLT]

Master cylinder. Make sure you bench bleed it before you install it.

[Nick Z]

What ended up being the issue? Was it the booster or MC?

[jescates]

Won't know until I get the new master cylinder in! I will post once I get the part and install it, probably next weekend!

[jescates]

Changed out the master cylinder today! Didn't have a vice or adapters to a bench bleed, so I installed it, jacked the truck up and got the MC as level as I could. Left the brake lines off and started pouring in brake flood. When it slowly started seeping out of the MC, I hooked up the lines. Then adjusted the brakes as tight as I could to get the brake shoes lined up, then adjusted the brakes back out. bled the brakes and now the brakes are perfect! Thanks to this website for all the information! Once again, another truck back on the road! Next step, rebuild the steering rag joint!