losing brake fluid, no visible leaks - GM vehicle

Okay, so Power Brakes haven't changed much in the last 50 years so I'm having this issue on my 1976 GMC motorhome.

In August, I paid the restoration shop for new brakes. They replaced about 50% of the hardlines, all the soft lines (from frame to wheel), combination block (where the two lines from the master cylinder distribute to the lines going to each corner), calipers, wheel cylinders, and the master cylinder. Essentially everything but the booster behind the master cylinder and a few of the hardlines inside the frame rails.

This was all about 1500 miles ago and up to two weeks ago, things seemed great. Two weeks ago I took her on a trip and the pedal seemed a little softer and going further to the floor than I remembered (although it was probably 3 weeks since I drove her prior). I stopped at the next town across from the auto parts store and checked the master cyl. Sure enough it was halfway down in the rear. I filled it up and continued. The pedal still felt the same though.

When I got home from the trip I checked and it had gone down some more. I filled it again and noted no wetness around the master Cyl. Last night I traced ALL the lines and inside the wheel rims (didn't pull off the wheels as jacking up 10,000 lbs is a PITA). I found NO indications at any of the six wheels or on the hard lines on any leaks....

As the front half of the Master didn't drop at all, I focused on the front lines more, but could find nothing.


Is it possible the when the new MC was put on the gasket to the booster didn't seat right and it's getting sucked into there and ultimately the motor?

Does it have drum brakes? It could be a leaking wheel cylinder and it's just not coming out of the drum area yet?

Leaking wheel cylinder, or... there was air in the lines that were replaced, and not properly bled after the service. That air eventually worked its way up and out.

Yes it has four drums in the rear and two disc up front (tandem rear wheels). Doubt it's the drums as that side of the MC stayed full.

Jay, I seriously doubt there was any air in the lines as
1. I know what that feels like when hitting the brakes,
2. The guy who did the brakes has been working on GMC motorhomes for over 15 years and I'm sure bled them properly.
3. Way too much fluid 'disappeared' to account for a couple of bubbles.

We got about 2-4 inches of rain today and more tomorrow, so I haven't been able to check anything else today, when it stops, I'll pull the vac hose off the booster and shove a white tie wrap in there like a dipstick and see if it's wet.....

leaking wheel cylinder

Maybe you lost a pad, or part of one off of one of the front calipers.

I would suspect the Master leaking into the booster.

Nick, everything is dry and it's only the rear half of the MC going down (disc brakes up front)...

Fred, I think I would hear the scraping and after topping off, more was lost and it's dry.

Pat, I tend to agree, which is annoying since the MC is new. While new it had surface rust so I assume it'd been sitting around and maybe the seals are dried out and leaky... I'll know by the weekend when the weather clears up (had a tornado about 1.5 hours ago about 2 miles from my house).

While it wasn't super wet, I pulled the master off. The booster and it was wet behind there where it should be dry. Going to guess my new MC sat in a shelf for a long time and the seals dried out... :-(