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I have a dual master cylinder to swap in, but it's quite a bit of work, and in the meantime I am using the original, single master cylinder. A little while ago, one of the wheel cylinders blew, and I replaced it. I then bled all the brakes. Now, I'm noticing, sometimes, the brake lever will slowly go all the way to the floor. I let it up, and the next time there is pressure. If I am at a stop light, and leave my foot on the brake, it usually, eventually, goes all the way to the floor. I replaced all the brake lines months ago, and when I crawl under the vehicle, I can't find any leaks. Anybody have suggestions?

Sounds like a leaky seal in the MC.

at this point, the master cylinder is one of the few things that hasn't been rebuilt or replaced. It might be 60 years old. What seal do you think might be leaking? And more importantly, you think it will be alright if I drive it around a bit this weekend? I can't replace the master cylinder until Monday at the earliest

I see no point in rebuilidng it if your intensions are installing the dually. Other wise you have to take it out 2x.
If you go that route, there are seals on the plunger shaft that could be bad or the cylinder itself could be pitted causing the leakage.
I can't suggest leaving it, but you could live with pumping the breaks till you get around to installing the new MC. Its not an uncommon thing to hear folks having to pump their breaks on these old rigs.

Driving in public with less the a 100% functional brake system is stupid. When that 3 year old kid steps into your path and dies while you are pumping up your brake pedal let us know how that makes you feel!

I'm just driving it around slowly, on local roads. And not through neighborhoods that have kids. I don't run around fast and loose with that vehicle as it is.

Local roads that are public use still have the same risk level. Picture the mom who is distracted and roles through a stop sign on your local road and while you are pumping up your brakes your bumper has impaled itself in the passenger side door . There's absolutely no reason to drive any unsafe vehicle on any public road.

I have a local convention that I have a table at. I am taking it there and back Friday through Sunday. 15 minute drive. Monday I'll replace the master cylinder. It's all last minute, I just rebuilt the e-brake, and at the slow speeds I'm traveling, that will stop me almost instantly. I'm not rich, and can't pay for a cab twice a day for 3 days.

You certainly can justify the operation of an unsafe vehicle to your self and your wallet but not to the general public and the victim. I hope you are very lucky this week for the innocent bystander's sake!

C'mon Wes. I'll be 40 this year. At this point in my life, I've had numerous older vehicles, and more than one have had brake problems. I had one even blow out completely, an hour from home, in the middle of the night, and I had to drive slowly using the e-brake to get it back to civilization. I called two of my mechanic friends, and they told me the brakes would be fine for the limited driving I'm doing this weekend. Like I said, slow, main roads but not highways, no neighborhoods. If the brakes were to blow out, and I used the ebrake, that would be it, I now have AAA so I'd have it towed, and use a taxi.

Dan, it's your call in the end what you do. You take what you want from the advise given and move forward. Wes is correct in what he is saying. Again, I wouldn't suggest it either. My previous post may of conveyed that I thought it was alright but I was just saying I have heard of people driving vehicles that way, I don't recommend it either. The brake system is there for a reason, to do its job.
Its up to you.

I'm stayin outa the way

The thing that can really give our hobby a bad name and cause legislation that will limit our hobby is people operating their historic vehicles when they have unsafe equipment. I really hope you reconsider.
Additionally, speaking as a state police officer and Disrict Attorney Investigator if someone did get hurt due to operating the vehicle when you knew the brakes were unsafe it would or should result in charges of aggravated assault or in the extreme negligent homicide or what ever the appropriate charge is in Pa.