Sudden brake loss in a 76 300D
 

The brakes have always been fine on this car till last night. The car has been parked the last 3 weeks ans when I started it the brake light was on and the pedal went all the way to the floor. I was able to get it into the garage where I topped off the brake fluid, (it did take most of the contents of a small can) that fixed the light indicator but I still had no brakes.

I saw no leaks from the master cylinder or anywhere else but the pedal would not pump up properly and hold so the brakes seem completely gone for the moment.

Anyone with any ideas what could be wrong?

If you're pumping with no pressure and no visible leaks, I'd say you're MC is leaking into the booster. Is the fluid level in the reservoir dropping?

It seemed I was getting some pedal pressure but it would immediately disappear. And I did not notice the fluid resevior going down.

I'd look for vacuum leaks then. The seal between the MC and booster is a common leak as is the main vacuum hose leading directly to the booster.

There could be several issues. One possibility is that when the fluid was low, it introduced air in the system. Another is you have a leak somewhere. If you can't see any leaks, then it could be what Chad suggested and is leaking into the brake booster.

The actual diaphram in the Booster can be cracked and leaking air also....

That's a good thought. He could try to bleed the brakes first. That's pretty easy to start.

I just replaced my booster due to this. Mine was leaking in such a way that it made a pop sound that I also felt in the pedal as though I had ABS. My pedal got firmer though so I think I may have offered incorrect info.

If you lose vacuum, the pedal would get very hard since the booster would have no vacuum to assist in driving the shaft in the MC.

I too am leaning toward air in the system being the cause based on the info shared so far.

The fluid lost originally had to go somewhere. If it isn't on the ground where the car was parked then it's probably in the brake booster. Has to be somewhere :)!

Appreciate the feedback from everyone. Turns out it was a master cylinder that had to be replaced a and a few worn brake line as well.

Job is done and everything works fine again.

Thanks again..