Brakes!! 1974 911

[914obsession]

I have a 1974 911. The old 19mm master cylinder was leaking pretty badly so 2 weeks ago I replaced it with a NOS 20mm I bought off ebay (I actually bought 2 of them). I also replaced the soft brake lines with some stainless lines. The brakes worked great for 1 week, then, with little warning the pedal began to get very soft and would actually go all the way to the floor. I checked the entire car for leaks, absolutely nothing, the MC did not leak and no fluid was missing from the reservoir. I bled the brakes again just to be sure, same problem.

So, I replaced the MC with the second NOS 20mm MC, everything worked great for 4 days, then today same problem. No leaks. The pedal went down to the floor a few times, if you pump the brakes a bit it will get firm again. Let the car sit for an hour and now the brakes feel fine. I can put all my weight on the brake peddle and it stays solid. Everything seems fine again, not too reassuring... Anything I'm missing? My next step would be to buy a new 19mm MC and start over again. I just can't believe that this is an MC issue, both 20mm were new and all rubber was perfect, pliable and the cylinder moved without sticking at all and no leaking.

Any advice would be extremely helpful.

Thanks!

[Steve@Rennsport]

You may have an MC issue.

If I may say this,...buying such things off eBay is pretty risky, given whats at stake here. I'd suggest that you source the right part from a reputable retailer that knows what they are doing and will stand behind their parts.

[914obsession]

I just can't see what would be wrong with the MCs (2 of them)? No leaking, it's a dual system that runs two pistons (one for front, one for rear) in tandem, if one of the piston's seals was bad you'd still have the other piston doing it's job. Both MC's are original ATEs new in the box.

Is there a proportioning valve in a 1974 911?

[JP911]

Sounds like bad seals in the MCs you bought, allowing fluid to pass through the chambers of the cylinder without creating pressure to the brake system. I experienced a failure like this on a 23mm MC after less than a year of use.