S2 Brake master cylinder reaction valve

[Hotrod]

Seeking guru with simple answer...I replaced my brake master cylinder using the new Lockheed 'made on the original tools'. I also had my servo rebuilt and have replaced all the vacuum hoses. The reaction valve now expels air with a hiss when you press the brake pedal with the engine running. The brakes still work once the hiss has happened. I removed the reaction valve and replaced with the one from the original master cylinder. Same thing happens. As far as I can see the diaphragm looks fine. Any ideas as to why this is happening? Things to check? Many thanks for your help. Rod

[MarekH]

Liquids (or gases) make a hissing noise when a high pressure goes through a small aperture. If the hole for the air to go through were bigger, then it wouldn't make a noise. If both of your reaction valves were the same, then they'd probably make the same noise as air rushes back in.

kind regards
Marek