1995 Ford Aerostar hydraulic leaks

Have had rear brake hardware kit, new brake master cyl, new rear wheel cyl installed. After the van was brought home I noticed there were two lights lit. The ADS yellow one and the red Brake too. Took it back in and the shop chief came out looked at the lights, opened the hood and looked, went back inside and came back with a bottle of brake fluid which he poured into the brake reservoir. I could see that there was not very much fluid in it, but he said that there might have been a "slight hydraulic leak" but he said there was enough fluid in it but when the fluid sometimes sloshes back and forth it probably triggered the sensor in the unit. He added some fluid and the lights eventually went off. I later found out from this site that there is a max fluid level mark on the unit and there was no way the amount of brake fluid was even close to that mark, so I filled it to the max level. Today I noticed that sitting at a stop light with my foot on the brake pedal, the pedal very, very, very slowly moves downward toward the floorboard. So I suspect there might be a small leak somewhere. My question is. If he thought there might have been a slight leak, shouldn't someone have checked for any leaks when they were doing the repairs?