So I won't mention what vehicle this master cylinder is on and I think I know my answer but it always good to get a second opinion. Similar to performing a full frontal lobotomy on yourself. Get a second opinion and not from the squirrel running around in your head.

So I've sucked out the old fluid from the master (I think I just gave myself away) and the brakes are hard and stops the wagon "ahem" car on a dime. But as I'm stopped with my foot on the brakes the pedal goes to the floor. I'm using no fluid. What is wrong with my wagon, er sorry car? 

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Master cylinder bypassing. Replace it.

Dang Russ you really know how to cheer a guy up. I was was hoping it was something I missed or a special kunnuter valve that needed adjusting. 

I messed it up by not putting a block of wood on the floor to stop peddle travel to the floor. It still would have needed to be done anyways. 

Sorry ol chap, but I have heard of not pushing the pedal all the way down on a used master cylinder. The crud is build up back where the piston doesn't normally travel, and you cut seals doing so.

Test it.  

Only two things can make a pedal fade without fluid loss;. trapped air in the lines or internal leakage in the master cylinder.    If the fluid wasn't disturbed or run empty, you can almost certainly rule out air in the lines.  If you're not sure about the fluid, you can test it logically.  

A quick check for air in the system is to have someone pump the brakes several times and then release the pedal.   If the fluid squirts in the air when the pedal is released, there is air in the system.  This happens because pumping the brakes will compress the air which will decompress quickly when the pressure is removed.   If there is only fluid, no compression can take place, so you will see only agitation in the reservoir with no squirt.

To test a master cylinder, get some plugs and install them in place of the lines.   Bleed the air and then check by pushing the pedal hard.   If it drops, then it must be the master cylinder is defective since you have isolated that component.   If it's hard as a rock, only moves a tiny amount and holds regardless of how hard you push it, it is ok. 

If neither test exposes the problem, u can pinpoint troubles with a pair of hose pinch off pliers.   Squeeze off each of the three rubber hoses one at a time at the front wheels and above the rear.  When the problem goes away, you know the trouble is in whatever the pinched hose supplies.

Finally, it isn't clear that what you describe isn't a bad vacuum feed to your booster.  You say the pedal is hard then drops slowly.   I'm not there to feel it, but that sounds like a sluggish vacuum booster that isn't building boost quickly enough.  So does the pedal fade with the engine off or only when driving?   If it's hard and steady when off, then the booster is at fault.

Enjoy

Now there's a great load of info. Thanks Mark. I have hydro boost not a vacuum system.

I'll give you a bit more back ground. 

I was doing some general maintenance and noticed the back master cylinder cup was almost empty. I topped it up and went looking for a leak. The brakes are at least 75 percent so that wasn't the problem.  I found the leak at the trailer brake controller under the dash. It's the old style with a line tapped into the front brakes. I took it apart and found the tiny hyd cylinder inside it leaking. I soaked the rubber cup in lacquer thinner and cleaned the old fluid out of it. All is good there now at least temporarily. I sucked the fluid out of the master and added fresh dot 3 and bled most of the old fluid out of the system including the trailer brake controller. My nephew did the pumping and I did the bleeding. Nice hard brakes and the trailer brake controller works better than ever. Sitting with my foot on the brakes and the hard paddle goes to the floor. My thought was that there was some gunk in the master and the extended travel of the peddle may have damaged the inner seals. Basically there may have been rust in the master just past its normal travel and I chewed up the seals by pumping them to the floor.

What's your thoughts with this added information. 

The brakes were fine prior to this event. They weren't as firm as they are now but 1970's firm and didn't feel unsafe. 

Would lacquer thinner not stuff the cup?

I'd be checking the stuff hasn't rooted the cup.

Brakes are for quitters... very overrated.

For you it is of course not necessary with brakes Kevin. As you are constantly up side down I guess you should worry more about falling off the planet. 

Of course, the symptoms are pointing to the master cylinder, but I don't like changing parts without seeing the failure first.   Take the master off and hone it, clean it like you did the trailer actuator, and replace the cups, they are too cheap to not just swap out.   

Mark, the rebuild kit was 34 and new master 70 at rockauto.com. I went with the new one, although they did have some rebuilt ones for 30 to 50 I decided to get a new one.

Kevin, I found an exact match to my brake controller on eBay, it's NOS. The one cleaned with lacquer thinner is still holding fine. I only soaked it long enough to soften it up and swell it a tiny bit. It was a stop gap repair to get me moving again but it is still holding up. Now I just want to see how long it will last LOL 

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