[Mahler9th]

I had this happen with a 951 way back when. I was supposed to drive in a weekend track event at Laguna Seca and the symptoms occurred that morning on the way to work that Friday morning, and then on the way down to Salinas after work I had to divert and go home instead. I missed the event.

What would happen is that I could drive a while and the brakes would start dragging more and more until it was clear that I needed to stop. Then I'd sit on the side of the road for maybe 20 minutes and start the cycle all over again. While I sat there I could smell the friction material. I was lucky that I did not ruin my expensive 965 rotors.

This was a highly-maintained street/track 951.

The culprit in my case was a leaky seal between the brake fluid reservoir and the booster. I discovered it on the next day (Saturday) right after lunch... I had checked a bunch of things and then gone for a drive. After a few blocks the problem re-occurred.

I stopped on the side of the road, popped the hood and used a small flat blade screwdriver to gently pry the check valve out of the booster. There was a whoosh and voila, the issue went away and I was able to drive far enough before it cropped up again to get home. When I got home I removed the master cylinder and used my Mityvac tool to see if I could suck fluid out of the booster. I got a few ounces. Problem identified. I quickly washed up and jumped into my other 951 and made it to Partsheaven just before they closed (3 pm). I bought a good used booster and installed it the next day.

I made sure to use a fresh master cylinder seal.

Never happened again.

Good luck.