The Stovebolt Site Forums The Shop Area Driveline Brake pedal goes to the floor

So I have all original brakes. A new master cylinder new brake lines and rubber flex lines. Took apart the brakes cleaned and set brakes . Then honed the cylinders. Did not replace the brake cylinders or rubber cushions as they seem to work fine now. Bled the brakes and pumped up the brake pedal. At this point the brakes work good. A few days later the pedal goes to the floor . I can pump up the pedal and again have brakes . Every few days of non use the same thing and I must pump up the brakes then it is good for a few days. What is going on. Where is my brake pressure going? Any help would be great. 1950 216 gmc 1/2 . Thanks

If you did actually remove all the air when you bled the brakes you are either leaking fluid or introducing air. Since everything is new except the wheel cylinders that would be the first place I would look. You may be leaking from the wheel cylinders and it may not be showing externally yet.

Hy can216, if you honed the wheel cylinders but did not replace the cups, you may have a poor seal in the wheel cylinders as Hookalatch said. This will allow the residual pressure in the system to bleed down quickly, once that happens air and moisture can be drawn in past the poor cup seals into the wheel cylinders, giving you a low pedal until you compress all the air and push it out along with some fluid past the poor cup seals. You should never reuse old cups when overhauling brakes, this system protects you and everyone on the road around you, it is not the place to go cheap. Why would you renew everything else and then reuse old cups and expect a good seal in honed cylinders?

I'd look at the shoe adjustment and make sure they are adjusted properly. use something like this Adjustment tool

Sounds good I think I will replace the entire wheel cylinders and be done with it. It seems to take forever to get parts shipped here. Canada eh!

In addition to what the others said, and this actually comes back to what Chuck posted, by draining the system completely, there is a very good chance you may still have air trapped in the master cylinder. You will have to go through the bleeding process a few more times than you would if you were simply replacing a line or wheel cylinder that does not completely empty the system.
To make this as easy as possible, I would suggest using a pressure bleeder. I have found that it's really the best way to completely bleed the system, especially from a dry situation.

When I bleed the system I use a garden sprayer. I replace the master cylinder cap with a plastic cap in which I have drilled and installed a hose barb to fit onto the 1/4'' line from the garden sprayer . Pump up the sprayer filled with brake fluid and then turn on the trigger. This pressures up the system and I can go around and bleed each brake cylinder one at a time or even all at once . I usually put a hose on the brake cylinder petcock to catch the discarded fluid. Easy and clean and no pumping and holding.

Ok. I use a pressure bleeder that is essentially the same style setup. That should work fine. You may consider adding a gauge to it if it doesn't have one, so that you make sure you have sufficient pressure to get a proper bleed. Around 40-45 psi is the optimal pressure for bleeding the system on these trucks as well as most non-abs systems.
I would definitely replace the cups in the wheel cylinders. You might try seeing fi they can match them at the local parts store, as wheel cylinder cups really aren't too much different over the years. It's more getting the right size.

I would loosen and tighten each and every brake line fitting several times to make sure that you don't have any leaks (fluid out, or air in). As you do this you will be amazed how much more they will turn compared to when you do it just once. This will give you a very positive fit that will not leak. It is VERY important to do this on stainless steel lines.
Carl

Along with what everyone else said....
I'd suggest a small investigation first. Pull the drums, got to do it anyway, and check for fresh fluid....and check all along the line. You may have a small pin hole in a line that is rusting from the inside out.

In my neck of the wood finding just the cups is almost impossible unless you have an old supply parts house in your area. Most will sell only kits but seeing how they are not expensive "any" kit would work as long as the cups are the correct size. Most cups will have the size on the inside of the cup.

There is another reason for pulling the drums. If you have a leaking wheel cylinder, you could get contamination on the shoes.

I would bet you have air getting sucked back in past one of the rubber cups you did not replace. I had a 1950 DeSoto years ago that I rebuilt the front Wheel cylinders (I honed them and put kits in them, new rubber!) and one of them would suck air in (top on driver side). I had brakes all day long, just fine. After sittin overnight, I had to pump them to get a pedal. If I opened the bleeder on that cylinder, and pushed the pedal down to the floor once, it let all the air out and then I had brakes the rest of the day till it st overnight! I wold replace all of the rubber cups or even the wheel cylinders.

Hy can216, the Napa Canada website shows these wheel cylinders as still available, however the website does not seem to allow access to pricing. Napa's p/n for the front wheel cylinders is 3406 their line code is NUP, the p/n for the rear wheel cylinder is 3396 again the line code is NUP, hope that helps.

Thanks guys this is good info. I'm off to napa to see what I can find. I will also do the retightening of the brake lines. Think I may have found the culprit. Drivers side back tire shows signs of fluid. I will start there.

Can216, ..... You mention 216 in your original post.....note....... 1950 GMC would have the GMC 228 in line six as the original engine.

The Canadian GMC had a Chev chassis including the 216 engine.

Ok