The Stovebolt Site Forums The Shop Area Driveline Brakes not working after sitting way too long.

After sitting too long with lines disconnected at the dual reservoir master cylinder w/o using, it seems the MC is ignoring the plunger. The pedal goes to the floor. It will bleed some fluid from the front and rear reservoirs but the pedal won't firm up. It just goes to the floor.

Check valve or something else stuck? Time for new MC? Just keep bleeding?

Thanks.

Try pulling the MC and bench bleeding it. Mount the MC in a vise, plug the ports, fill the reservoir, and gently work the piston in and out about 1/2" or less until bubbles stop showing up in the reservoirs. Then reinstall it and bleed the rest of the system.
Jerry

I am curious about your bench bleeding procedure. I learned to put a loopback from the port to the reservoirs and pump it until no more air came out.

You have been doing this long enough that your way clearly works, but I am curious why you block the ports? Doesn't require the bubbles to work backwards through the mechanism?

Recirculating bench bleeding went out of favor around 15-20 years ago. It's difficult to even find the little plastic tubes with the double thread fittings these days. Now new or rebuilt master cylinders come packed with two plastic plugs instead of the recirculator tubes. It's quicker, simpler, and a lot less messy to simply plug the ports and let the fluid displace the air in the cylinder bore by letting the air come back up through the vent ports while short-stroking the MC.
Jerry

I just cut a short section of brake line and bent it back to the fill plug. Easy enough to do. I do recall the plastic plugs with my new master clyinder, but they weren't threaded, just push-in, and wouldn't do any real sealing.

I decided to go with a "new" MC. It came with threaded plugs. I had some old fittings with tubing so I used those.

My new Wilwood master came with a multi-tubing bleeder kit.

Wish you had posted this a couple of months ago - could have saved me $10. When I picked up the new master cylinder, I looked in the box and asked the parts guy why there were no tubes for bench bleeding. He didn't know why so he sold me a kit with the plastic tubes and fittings.

Like Smith Barney, I did it the old fashioned way.

Getting the air out of the MC bore is necessary, no matter how it gets done. Since there's no mechanical connection between the pistons in a double reservoir master cylinder unless one end fails, bench bleeding can be a challenge. Usually one chamber will purge itself of trapped air before the other. Then there's a hydraulic connection instead of a mechanical one to make the second piston move. When one piston or the other loses its ability to push fluid, or there's a leak downstream that empties one reservoir, then the pedal drops by about 2/3 of its travel, the pistons in the MC touch each other, and the intact fluid circuit does all the braking. If a constant gentle pressure on the brake pedal causes it to fall away slowly, that's an impending master cylinder failure warning you to do something about it.
Jerry