Two years ago I converted the hydraulic brake system in my "88 Vanden Plas to conventional brake booster. Occasionaly (about 1 in 20 times) when applying the brakes the pedal will give way and go almost to the floor. Brake fluid is at the proper level. Could it be an issue with the master cylinder, air in the system, or something else? Thanks in advance.

 

My first suspicion would be the master cylinder if it's been fine there 2 years, then air is unlikely.

 

The Master Cylinder is original and there was a good possibllity it was leaking and letting in air, so I sourced a new rebuild kit and replaced all of the internal parts with new ones from the kit. I had the brakes bled afterwards, and now they are as good as ever.